Jim Harris has gone one up on people who wear their hearts on their sleeves; he carries a kind of pancreas attached to his belt. A 27-year-old department store employee from Palm Springs, Calif., Harris is one of only a handful of the nation's 10 million diabetics to be outfitted with a 10-ounce, battery-powered blue box known as an insulin-infusion pump. The device, which approximates the on-demand insulin-producing action of a healthy pancreas, releases a steady trickle of insulin into his bloodstream through a tiny needle he inserts daily under the skin of his abdomen. The happy result for Harris is that he is free of the twice-daily injections he has been administering to himself for 16 years.

For the privilege of testing this extraordinary apparatus, Harris is indebted to Dr. Richard J. Mahler, 47, chief of the Department of Endocrinology and Metabolics at the Eisenhower Medical Center outside Palm Springs. Though a veteran diabetes researcher, Mahler is not the first or only doctor to investigate the pump's potential. Several studies have already demonstrated that a continuous supply of insulin will help maintain a diabetic's blood sugar at a normal level. What Mahler hopes to learn is whether the pump can prevent or reverse such long-term complications of diabetes as bloodvessel damage leading to heart attacks, strokes, blindness and gangrene. "Jim was the ideal patient for the study," Mahler explains. "He'd had diabetes more than 10 years, and he'd started to develop some complications, such as thickening in the blood vessels."

The pump, which costs $800 and was developed in England for use in chemotherapy, holds a two-day supply of insulin and can be easily refilled.

Before each meal, Harris increases the flow of the life-sustaining secretion by turning a small dial on the side of the pump. Four times daily he measures his blood-sugar level with a portable device called a Dextrometer that analyzes blood samples extracted from the body. Mahler hopes that by coordinating the flow from the pump with Dextrometer readings, diabetics will be able largely to eliminate the uncertainties of dosage and timing that are involved in the customary injections. In the 10 months he has used the pump, Harris' blood-sugar readings have been consistently normal, except for once, after a cheesecake "pig out," when he forgot to increase his insulin supply. Though the results are preliminary, Mahler calls the new treatment "the biggest breakthrough in diabetes control since 1922, when insulin extracted from a dog and injected into a human being was shown to lower blood sugar. I know how hard it is to sell a diabetic patient on doing all the things he has to do to control his disease," Mahler adds. "If we show we can actually prevent or reverse long-term complications, I think patients will be more willing to help themselves."

Harris is the first of six volunteers who are expected to take part in Mahler's two-year, $90,000 study. In December researchers will examine tissue samples from Harris to see whether the constant infusion of insulin has halted the dangerous deterioration of his blood vessels. For the dedicated Mahler, who opens his office at 6:30 a.m. and rarely arrives home before 10 p.m., such a finding would be a sweet vindication. Aside from one month a year, during which he travels with his wife, Ida May, and their two children, Mahler's life is totally devoted, he says, to "treating, teaching and doing research."

As for Harris, he has grown accustomed by now to the curious stares of customers in I. Magnin's Palm Springs store, where he is in charge of the men's clothing department. "They think I'm wearing some sort of inventory-control device," he says with a shrug. And in a way, of course, the customers, as always, are right.