Effects of diltiazem, a recently introduced calcium antagonist, on exercise performance were studied in nine coronary disease patients with effort angina. The duration of exercise before the onset of angina and the time to the onset of ischemic ST depression 2 hours after 90 mg of oral diltiazem were compared with those 2 hours after oral placebo and a few minutes after 0.3 mg of sublingual nitroglycerin. Diltiazem prolonged the duration of exercise in all nine patients (average 2.5 minutes, p less than 0.001) and delayed the onset of ischemic ST depression (average 2.4 minutes, p less than 0.001). The increment of the duration of exercise and the time to the onset of ischemic ST depression following 90 mg of oral diltiazem were almost equivalent to that following sublingual nitroglycerin. These results in fixed coronary atherosclerosis indicate the clinical antianginal efficacy of diltiazem which persists for at least 2 hours after oral administration.