Mr. Fixx's case ''will be a big question raiser'' because of the irony of his death while jogging and the debate about the health benefits of exercise, said Dr. Robert S. Ascheim, a cardiologist who practices at 435 East 57th Street and teaches at New York Hospital-Cornell Medical Center.

''Does running benefit you? Nobody really has a clear answer,'' Dr. Ascheim said.

Nevertheless, many people believe exercise prevents death from premature heart disease. Dr. Ascheim said that the severity of Mr. Fixx's heart disease would not have made bypass surgery imperative but that national studies have shown that medications would have been advised.

As the debates go on, optimists will say that exercise prolonged Mr. Fixx's life, and pessimists will contend that it shortened it.

Because of the uncertainties, one of the thorniest questions physicians face is how many tests and which ones to advise to screen for heart disease in middle-aged males who have no symptoms. Many physicians would not have been particularly suspicious of Mr. Fixx's having heart disease because he was in such fine physical condition and because of the apparent absence of symptoms even when exercising strenuously.

A physician examining him would start with a medical history, asking such questions as whether the patient has felt pain in the chest, jaw, throat or arm in relationship to exercise. He would ask the patient to describe the character of the pain and to tell how long it has been present. Newly occurring pain might merit special attention.

On learning that 17 years had passed since Mr. Fixx stopped his two-pack-a-day smoking habit and began running, the physician would have assumed that his risk of a heart attack would have returned to that of a nonsmoker. However, the physician would be struck by the heart disease history of Mr. Fixx's father.

If Mr. Fixx's blood pressure was high, appropriate therapy would have been advised.

Mr. Fixx's heart, like those of many athletes, was enlarged, and it probably would have appeared so on a chest X-ray. But, as Dr. McQuillen said, ''it is hard to know what the significance of an enlarged heart is in a runner'' who had serious heart disease.