Breathes there an endomorph who has never given in to the fitness syndrome and tried exercise, then gasped and wheezed and decided it's too late, the reward is not worth the pain?

It turns out some of them need not feel guilty. They are asthma sufferers, even though they've never had the usual asthmatic's allergy to pollen or dust or other air pollutants. What they are allergic to specialists have discovered, is exercise.

Doctors have found that in some people physical stress causes bronchial tubes to constrict and the struggle to get sufficient air through the narrowed passages brings on puffing and wheezing and gasping.

The panic induced by the condition often aggravates it and makes it last longer than the shortness of breath suffered by the ordinary out-of-condition person after extraordinary activity.

These people suffer from what has been labeled exercise-induced asthma, and they are a significant portion of the estimated 6 to 8 million asthmatics in the country.

Exercise-induced asthma is common in children, though not often diagnosed as such, and it can turn up in adults who never had breathing problems in childhood, according to Dr. Richard R. Rosenthal of Fairfax, who specializes in asthma and allergies. Even athletes in the best of condition are not immune.

Janet Lynn, who skated her way to the bronze medal in the 1972 Winter Olympics, didn't develop it until 1974. That year, when she was 20, a severe bout with penumonia forced her to cut back her professional appearances with the Ice Follies. The following year her skating caused asthma attacks.

Rick DeMont, the 16-year-old swimmer who had his gold medal taken away after he won the 400-meter freestyle event in the 1972 Olympics, has it too. He was disqualified a conventional drug used in treating asthma before the race.

What is the difference between the shortness of breath most people feel after prolonged exercise and the sensations of a genuine asthma attack? Usually it is in the intensity of the symptoms and the duration of the attack.

According to Dr. A. Larry miller, a Vienna, Va., allergist, the person with exercise-induced asthma will continue to gasp for breath, wheeze, cough and sputter for up to an after exercise.

"The reaction may occur during the exercise," says Miller, "but more commonly it starts from 5 to 10 minutes after the exercise has been completed. THe symptoms generally subside within an hour."

These people aren't necessarily bothered by pollen, house dust, or any of the other substances that commonly stimulate an asthmatic reaction. It's usually just stress put on the body's airways during exercise.

Exercise-induced asthma attacks also may be triggered by exposure to very cold air or extended laughter.

Despite the fact that the link between exercise and asthma isn't new, countless people suffer from the disease without realizing they have it.

The problem is that since it may show up only when the body is under physical stress, doctors have often been unable to diagnose it in routine office visits. They may have to rely on the patient's description of his symptoms and his past history, and frequently the exercise connection is overlooked, physicians say.

Progress has been made in the last 10 years and now doctors are generally more aware of the phenomenon. There are more sophisticated instruments to measure breathing in pulmonary labs, and drugs have shown promise in treating asthma.

Dr. Rosenthal said drug research in this country and Europe since the mid-1960s has resulted in development of a whole new class of drugs that reduce side effects associated with conventional asthma therapy, and in many cases are used successfully with exercise-induced asthma. Rosenthal said he is excited about the research, but still considers it "pharmacology in its infancy."

Rosenthal does his research at Johns Hopkins' Asthma and Allergic Disease Center in Baltimore. The center is one of 15 of its kind in the country, federally funded through the National Institute of Allergy and Infectious Diseases.

Rosenthal and his associates measure, with the help of electronic instruments, the lung capacity and breathing patterns of the patient at rest. Then, as the heart beat is being monitored, the patient runs for five minutes or more on a treadmill. The measurements are taken again, at intervals, until breathing is back to normal.

The procedure is then repeated, but this time the patient inhales one of the new drugs before boarding the treadmill. The change may be dramatic. The patient who was gasphing for breath after the first go-round may feel almost the way the average person does after vigorous exercise.

"The hallmark of asthma is reversibility," Rosenthal said.

His advice: If you suspect you have exercise-induced asthma, see your doctor. There's growing hope for a whole group of people who may have thought they were simply horribly out of shape.