The caffeine in just two or three cups of coffee makes the blood pressure shoot up. It slows down the heart, then speeds it up. It speeds up breathing. It forces the adrenal glands to pour out two hormones that make the nervous system work harder.

All these striking, potentially harmful effects have been detected by Vanderbilt University scientists in one of the most careful studies of caffeine effects ever done.

The researchers also concluded that medical science doesn't know yet whether habitual coffee-drinking increases the risk of developing heart diseases or other illness, but investigation is needed.

As for advice to the average coffee user, "it's too early to make any recommendations," the study's principal author, Dr. David Robertson, said yesterday.

Many other authorities agree. But not all.

Dr. William Lukash, President Carter's physician, said yesterday "it's fairly well established" that caffeine is a potent drug, "and I'd use a common-sense approach to limit coffee intake to no more than three or four cups a day."

Lukash and two Navy colleagues told the American Medical Association in 1975 that drinking 5 to 10 cups a day can lead to headaches, irritability and nervousness. They said 30 percent of coffee drinkers "have digestive distress directly attributable" to their coffee use.

All authorities agree that coffee also should be limited in certain medical patients - for example, persons "with any heart rhythm disturbance," said Robertson.

He and six Vanderbilt colleagues tell in the New England Journal of Medicine issued today oft heir unprecedented study of caffeine effects in nine healthy young persons - six men and three women - aged 21 to 30.

All ordinarily were non-coffee-drinkers, and none was allowed any coffee, tea, cola drinks or drugs for three weeks before the experiment. All had their salt intake controlled, since excess salt can affect some of the same factors.

Rarely if ever has anyone studying coffee or caffeine effects so carefully made sure they were studying these effects alone. Tea drinking was barred because tea also contains caffeine - a half to two thirds as much per cup, Robertson estimated.

At various times on two separate days, without knowing what they were drinking, the nine subjects were given doses of caffeine in an otherwise harmless beverage or a caffeine-free drink.

After they consumed the caffeine, it was found that, on the average:

Their adrenalin output increased by 207 percent and the output of a related chemical - norepinephrine by 75 percent.

Blood pressure increased by about a tenth.

Breathing rates increased by 20 percent.

Heart rates dropped slightly at first, then increased after about an hour.

Many of the effects lasted for about three hours of observation. Most of the subjects also showed a sharp increase in output of renin, a kidney enzyme commonly found in greater amounts in persons with high blood pressure.

In 1973 Boston University researchers studied the coffee habits of 12,759 hospitalized patients, including 440 with acute myocardial infraction (heart blockage). They said drinking one to five cups of coffee a day had apparently increased heart disease risk 60 per cent, and drinking six or more cups, 120 per cent.

But other studies have cast serious doubt on this finding.

It is possible, said the Vanderbilt group, that caffeine raises some susceptible persons from normal to high blood pressure. It is also possible, said Robertson - now spending a two-year period at Johns Hopkins University - that "moderate coffee drinking doesn't really result in a great difference in health."

Robertson's own coffee intake is one or two cups a day.