Humans who have benefited from medical treatments perfected through animal research number in the millions. They include the 110 million Americans under 30 who have never had to fear polio epidemics, the 1.5 million diabetics who live with insulin, the 5,000 individuals who undergo successful kidney transplants every year and the handful of pioneers who have received artificial hearts. Indeed, any person who has ever taken a prescription drug or been vaccinated has reaped the benefits of animal research.

The following diseases are under study by medical researchers using animals in their pursuit of more effective treatments and possible cures:

*Coronary heart disease. Studies in frogs, reptiles, horses, cats, dogs, sheep and deer have contributed to scientists' understanding of the fundamental principles of circulation, blood pressure and temperature regulation, all critical dimensions of coronary heart disease -- the leading cause of death in the United States. Years of research using animals led to the refinement of coronary artery bypass graft surgery, now the most commonly performed major operation in the United States.

*Heart transplants. Researchers at Stanford University, led by Dr. Norman Shumway, developed heart transplantation procedures through thousands of animal experiments, primarily in dogs. This procedure, which has been performed in some 1,000 individuals, now is accepted worldwide as a reasonable therapeutic option in patients with terminal cardiac failure. The expected one-year survival rate of individuals who undergo heart transplantation is 80 percent and the five-year survival rate is about 60 percent. By comparison, the six-month survival rate of patients accepted for transplant but not operated upon is zero.

The implantation of the artificial heart in Dr. Barney Clark almost three years ago was the culmination of decades of experience in thousands of long-term implantations in calves. Current research now under way at the University of Utah centers on developing a much smaller power pack for the artificial heart.

*AIDS. Researchers at the New England Regional Primate Research Center believe that monkeys may provide the critical link in the search for techniques to treat and prevent acquired immune deficiency syndrome. The chemistry of a virus found in some of the center's macaque monkeys resembles the agent that is believed to be the cause of AIDS in humans, according to Dr. Ronald C. Desrosiers. The finding is significant because no animals that doctors have infected with human AIDS have ever developed the full-blown disease, he says. Because the center's scientists have isolated the virus and have the capability of inducing it in other monkeys, these animals can be used as a test system for looking at the best approaches for AIDS vaccines and drugs, he said.High blood pressure.

*Untreated, hypertension increases the risk of stroke, heart disease and kidney failure. Monkeys are used to examine mechanisms of high blood pressure because the natural hormone molecules controlling blood pressure are identical in humans and other primates.

*Parkinson's disease. This neurological disorder of older adults is characterized by palsy and rigid muscles. In 1983, the first animal model of the disease was developed. Scientists at the National Institute of Mental Health induced a form of parkinsonism in eight rhesus monkeys by giving them a drug that selectively kills specific cells in a region of the brain destroyed in humans by the disease. NIMH researchers have speculated that the development of this new animal model may shed light on why Parkinson's disease occurs in older adults and how the disease progresses. It also will make it easier to study drug therapies and their side effects.

*Baldness. Like many men, stumptail monkeys become bald as they age. This trait has made the stumptail monkey the animal of choice in baldness research. A drug originally developed to manage high blood pressure, minoxidil, has the unexpected side effect of causing thick hair growth from follicles that normally produce only fine hair. To test the potential of the drug to induce hair growth, researchers applied it externally to the bald front scalp of stumptail monkeys. The results with monkeys showed promise, and clinical trials are now in progress with bald men across the country. Alternatives

While the research community regards the use of animals as fundamental to much of its work, efforts are under way to develop alternatives.

Alternatives to animal use in biomedical, behavioral and veterinary research fall into four categories: continued but reduced use of animals, including efforts to minimize pain; use of living components, such as cell, tissue and organ culture, instead of the whole animal; use of nonliving systems, such as chemical or physical systems and computer simulation. At the request of Congress, the Office of Technology Assessment is winding up a comprehensive survey of existing alternatives to animal use.