It's batting practice at the Seattle Kingdome, and Ken Griffey Jr., the Mariners' hotshot rookie, is putting on an aerial show. Crack—he rattles one off the wall. Crack—he blasts one into the bull pen. Crack—the 19-year-old center fielder, the youngest player in the major leagues, rips a tape-measure job into the seats.

It's the most impressive power display this side of Hoover Dam, and taking it all in, from behind the batting cage, is the kid's proud papa—39-year-old Ken Griffey Sr. "I never had any idea this would happen—I never pushed him," says the elder Griffey, who plays first base and outfield for the Cincinnati Reds. Although many major-league fathers have sired ballplayer sons, Griffey pére et fils are the first father and son ever to play in the majors at the same time. And Griffey Sr., now in his 16th season, couldn't be happier. Tm just thrilled," he says.

So are the Mariners. Baseball savants have already proclaimed the 6'3", 195-lb. Griffey Jr. the game's next superstar. With his average hovering around .280, he already has 11 home runs and 34 runs batted in while fielding his position like a young Willie Mays. "He does it all," says Jim Lefebvre, the Mariners' manager. "And he does it so fluidly. That's the mark of the great ones."

If the kid seems more polished than most rookies, it's no accident. He grew up in Cincinnati, where his father, a lifetime .298 hitter, broke in with the vaunted Big Red Machine. "I watched my dad play for years," says Ken Jr. "I talked to him every day about the game. There isn't one thing I've seen so far that he hasn't told me about beforehand."

Their baseball relationship went beyond talk. During the five seasons Griffey Sr. played for the New York Yankees, he would take Ken Jr. with him to the batting cage and feed him a challenging diet of fastballs, curves, change-ups, even sliders. It didn't take long to see that his boy had talent. "After age 12," says Ken Sr., "I couldn't strike him out."

Ken Jr. starred in football and baseball for Cincinnati's Moeller High and was the first of 1,263 players chosen in the 1987 baseball draft. He immediately joined the Mariners' Bellingham, Wash., farm team—and went into a major-league tailspin. Just 17 and away from his family for the first time, he became as depressed as his .230 batting average. So he called his mom for help. The next day Birdie Griffey, 38, flew in for a motivational seminar. "I knew he needed some sympathy," she says. "But I got mad and told him to concentrate on his career." Chastened, Ken Jr. went on a hitting tear that raised his average nearly 100 points.

Since then, Griffey has discovered that family-style support needn't come only from relatives. He lives alone in a luxury Seattle apartment, but his fellow Mariners are always watching out for him. "He's like a little brother to most of the guys," says second baseman Harold Reynolds. "Anyhow, he can't get into trouble. He's on the phone all the time."

His monthly message units, in fact, are as impressive as the rest of his stats. "I call my mom every night," admits Ken Jr. "And my dad, too. He tells me to call him whenever I want."

"He's still a kid," says Ken Sr., laughing. "Of course, he reverses the charges." Ken Jr. also keeps in close touch with his 21-year-old girlfriend, Missy Parrett, who lives in Cincinnati, and with his brother Craig, 17, who's headed to Ohio State on a football scholarship.

Crack. Back at the batting cage, young Griffey makes quick work of another gleaming new baseball. Griffey Sr. can't help but beam. And then, giggling like a kid, Ken Jr. breaks out of the box and into a goofy home run trot. "Touch every base, touch 'em all," he shouts.

"Behave yourself," scolds his dad, "or I'll tell your mother."

—Jack Friedman, Nick Gallo in Seattle