For 26 years Leonard Nimoy has struggled to establish himself as an important American actor. But in the minds of millions he is unshakably identified as Mr. Spock, the green-blooded, pointy-eared extraterrestrial who serves as first officer of the starship Enterprise on the eternally rerunning TV series Star Trek.

This summer, nevertheless, Nimoy moved into the Broadway production of Peter Shaffer's Equus and took over the eloquent, deeply searching role of Dr. Martin Dysart, an alienated alienist treating an unstable stable boy who thinks God is a horse. Would Nimoy manage at last to get out from under those peculiar ears?

More than image was against him. For one thing, his performance would be compared with the work of four famously skillful actors who preceded him in the part: Alec McCowan, Anthony Hopkins, Tony Perkins and Richard Burton (who plays the psychiatrist in Sidney Lumet's forthcoming film version of the play). For another, Equus has been running for nearly 1,150 performances. With Perkins leaving, Nimoy was asked to take over and make sure the old gray production kept on being what it used to be.

No small challenge, but Nimoy is not a man who swerves from challenges.

Even before rehearsing the first scene of Equus, Nimoy made a scene of his own. Small eyes glittering, long upper lip set hard, he was frightening to look at and he meant to be. He wanted the play to take new roads and he was ready to drive it along with any stick at hand. What concerned him most fiercely was what happens between Dysart and his patient, played by a gifted 21-year-old actor named Ralph Seymour. To stir up the relationship, Nimoy decided to rip into Seymour as savagely in real life as Dysart rips into the boy in the play.

"You smoke too much, friend!" he snapped less than five minutes after they met. "Your part demands a powerful voice but your voice is shot. I didn't come to New York to play around with a pussycat. You'd better be up to this part or you're gonna bleed!"

Staggered, Seymour quit smoking on the spot. His voice improved and, onstage as off, he kept a wary eye on Nimoy—the very attitude the part requires. Then Seymour began to sneak a drag now and then; one day he showed up with incipient bronchitis. In a rage, Nimoy stopped rehearsal and ordered Seymour to a throat specialist. "Are you smoking again?" he demanded. When Seymour was evasive, Nimoy looked him in the eye and said, "You're an ass," then strode offstage.

Nimoy was just as hard on himself. At the theater he rehearsed seven hours a day; at home he rehearsed several more; and all night, as his wife testifies grimly, he thrashed about the bed rehearsing in his sleep. He questioned every line, nudging the other actors to react anew to his fresh insights. Even director John Dexter, benumbed by the experience of four very strong-willed Dysarts, was caught up in the creative vortex and developed an elementally new vision of the role. By the time Nimoy's opening night arrived, the production was in furious gestation. When the house lights went down, only God and the stage cat knew what might be born.

Nimoy has always been a determined cuss. Born in Boston 46 years ago, he was an intense, pent-up boy, more like his forceful mother than his easygoing barber father. At 8, he showed an inkling of dramatic talent while playing the lead in a settlement house production of Hansel and Gretel. At 17, his dignity infringed when pretty girls passed him over in favor of high school jocks, Nimoy vowed to "get revenge" by becoming a famous actor. Defying his practical parents, he took $600 earned by selling vacuum cleaners and, after three junk-food-filled days on a transcontinental train, enrolled as a student at California's Pasadena Playhouse.

Nothing came easy. Nimoy worked so hard to break his Boston accent that he developed a temporary stutter. And his lean, saturnine looks set limits to his hopes: "I never dreamed I could be a star. I just wanted to do good work."

After a dozen grim years he had done little more than bit parts in four laughably bad movies and the lead in a fifth (Kid Monk Baroni), plus an occasional minor TV role. Cruel times for a young man who in 1954 had married an attractive actress, Sandi Zober, and soon had two babies.

Nimoy worked in a pet shop, jerked sodas, delivered newspapers, ran a vending machine route, drove cabs at night. But he hung on tight to his dream, and Sandi backed him up. "We had terrible fights," she remembers, "sometimes because he wanted to give up acting and take a sensible job and I wouldn't let him. Leonard wasn't much fun in those days, and I didn't always appreciate what a strong husband and father he was. But we worked it through."

In the early '60s Nimoy at last began to get big parts on TV shows. In 1965 he was cast as Mr. Spock in Star Trek. Half Vulcan and half Earthman, Spock in the early scripts was a sci-fi cliché: a mysterious hemihuman with a giant brain and no feelings. But Nimoy, seeing in Spock a secret longing for love, badgered his scriptwriters to exploit the dramatic possibilities.

When they did, Nimoy played like a bottled cyclone. The sacks of Spock mail reached 10,000 letters a month, mostly from women, much of it torridly erotic. Leonard Nimoy fan clubs sprang up all over. The name LEONARD NIMOY appeared on his very own parking space. He loved it.

Nimoy fought hard for better stories and better lines, mostly with his producer but sometimes with his co-star William Shatner. Says Shatner: "We went through that fire together and today we are fast friends. Leonard is an honest man and a fine craftsman."

Worn down by constant battles, Nimoy wound up on an analyst's couch. Then NBC grounded the show after three seasons, though it threatens to rerun until the universe crawls back into its little black hole. (Lately Nimoy had agreed to appear in a Star Trek movie but not in the revived TV series Paramount is now contemplating.)

In the interim he signed a four-year, $7,500-a-week contract with another long-running series, Mission: Impossible, but two years later abruptly quit. Well off, he decided he was tired of playing roles he didn't believe in for money he didn't need. "In 1971," Nimoy says, "I had reached professional menopause. It was time for an artistic change of life."

But not for a rest. Discovering a green thumb, he tore up his patio in Los Angeles and replanted it. Photography came next. He bought a good Japanese camera and set up his own darkroom. Eighteen months later he put together a book of photographs captioned with unpretentious, sometimes moving poems of his own composition. A sample:

I have killed time.
I have squandered it...
As a man of unlimited wealth
Might drop coins on the street
And never look back.

Nimoy's third volume of verse (We Are All Children Searching for Love) will soon appear, and he has also published a graceful autobiography (I Am Not Spock). He learned to fly a plane (he has an instrument rating) and, encouraged by his assertively liberal wife, campaigned in 35 states for George McGovern.

Then in 1975, at 44, Nimoy entered college. This summer, after two years of study carried on during dozens of acting assignments ("I chewed up the whole elephant," he says cockily), he won a master's degree in education from Antioch College.

All the while, Nimoy kept hoping for a major movie offer, but it never came. What was the alternative? Theater. Nimoy discovered that dozens of great parts were his for the playing in regional theaters. The pay was often no more than $300 a week, but the psychic income was limitless.

Nimoy became a gypsy actor and the wife and kids often traveled with him. Much of the time Julie and Adam were educated by tutors—with obvious success. She is about to graduate from the University of California at Santa Barbara as an art major; he studies political science at Berkeley and has a summer job in the office of House Speaker Tip O'Neill.

In the six years before Equus, Nimoy played 13 major roles in 27 American cities. He was an exultant Tevye in Fiddler on the Roof, a prissy Malvolio in Twelfth Night, a raunchy McMurphy in One Flew Over the Cuckoo's Nest. And in Man in the Glass Booth, as a Jew on trial for collaborating with the SS, he gave what director Ben Shaktman of the Pittsburgh Public Theater calls "the most dangerous performance I've ever seen—a rattlesnake in its death throes, but you were never quite sure if it was dying or feigning."

Shaktman adds: "This man has very big range and dramatic intelligence. And the more he plays the stronger he gets—he's adrenalized by the act of acting. There'd be heart attacks in the front rows if he played lago—and he can do comedy too."

WANTED: Horse psychiatrist to help in research for Equus. 221-6687.

Believing that the play depicts a battle between modern man and a primeval power expressed in the Horse God, Nimoy made a simple, shrewd move to get inside the head of genus Equus. His ad in the New York Times was answered by more than 200 psychologists, veterinarians, trainers, jockeys, horseplayersand people who just whinnied and hung up. Nimoy hired an able young ethologist named Hedy Strauss as his instructor. "I came away," Nimoy says, "with a feeling of awe at the power of the horse in the night mind of man." Would that feeling help him play his part?

"Probably. A character is like a plant. The richer the soil, the better it grows. One of the actor's jobs is to nourish his plants.

"I started out as an organic actor, a Method actor. Playing Spock helped me personally to become more cerebral and objective. My feelings don't rule my acting now. I use what works.

"My job is to be part of a magical illusion created equally by the play, the players and the audience. When all those elements meet in the right way, the truth takes place between us and within us, and it's an experience like no other I know."

No amount of preparation can guarantee that experience. Would it happen in the Helen Hayes Theater on opening night?

On a raised square in a cone of light a lean unillusioned shrink paced wearily in a rumpled tan suit as he declaimed in hard clinical tones. From the apron to the upper tier not a breath was drawn. Nimoy had started slowly, too slowly; but the tension had tightened twist by twist as the audience sensed that Dysart and the boy were playing a fearful game in which the winner might save his sanity but lose his soul.

"Screw it!" director John Dexter burst out in high excitement at intermission. "I only came for the first act but I'm staying. It's good!" In the second act there were rough spots, but in scene after scene Nimoy chipped away insidiously at the ground of everyday reality under the spectators' feet.

At curtain call, the audience rose and cheered. Almost every night since his opening, in fact, the new Dysart and the revitalized Seymour have taken a standing ovation. Almost every night his performance has strengthened. Ticket sales for Equus are up 20 percent. Asked if he was satisfied, Nimoy let out a booming laugh. "Hell, no! I'm working harder than ever. The day I'm satisfied, I'm finished."