A study is made of 525 consecutive treadmill exercise tests. Horizontal or downsloping ST segment depression of 1 mm or more was considered or ischaemic response. A systolic blood pressure exceeding 200 mm Hg or a diastolic blood pressure exceeding 100 mm Hg during or after exercise was defined as a hypertensive response. An ischemic response was found in 48 subjects. In this group, 7 (15%) had previous myocardial infarction, 10 (21%) had minor ischaemic changes in the resting electrocardiogram. Twenty-eight (58%) complained of precordial pain or discomfort. A hypertensive response occurred in 101 subjects. Thirty-eight of them had precordial pain or discomfort. The resting blood pressure was elevated in 52, borderline in 32, and normal in 17 subjects. Cardiac arrhythmias were detected in 53 patients. Other abnormalities detected included 3 cases of hypertrophic cardiomyopathy and 1 case of post exercise bronchospasm due to propranolol.