THE FOLLOWING CASE is presented in order to introduce a note of caution into consideration of the advisability of recommending unmonitored exercise for patients with heart disease. The patient is a 24 year old third year medical student who had been known to have a heart murmur since birth. He had lived a vigorously active life and had participated in high school football as well as snow skiing in more recent years. At the age of 19 he underwent a detailed cardiac work-up including right heart catheterization at another hospital. The findings at that time included a normal examination except for a grade III/VI blowing pansystolic murmur at the 4th left intercostal space. The chest x-ray disclosed a normal cardiac silhouette with normal pulmonary vascularity. The electrocardiogram disclosed right bundle branch block and left axis deviation. Cardiac catheterization disclosed systolic pressure of 30 mm. Hg. in the right ventricle and 23 in the pulmonary artery. There was O2 saturation of 78 per cent in the superior vena cava and 75 per cent in the inferior vena cava. In the high and mid right atrium O2 saturations ranged from 76 per cent to 80 per cent. Low in the atrium, just above the tricuspid valve, there was a small step-up of O2 saturation to 83 per cent and similar saturations were found in the right ventricle and pulmonary artery. The findings suggested a small shunt from left ventricle to right atrium of the type seen in the transitional form of atrio-ventricular communis. During the two weeks prior to the present event the patient and his roommate who was also a third year medical student, had undertaken to improve their physical conditioning by performing the Canadian Air Force series of calisthenic exercises.