Twelve patients with hypertension who did not become normotensive after treatment with hydrochlorothiazide alone were studied. Treadmill exercise testing was done before and after additional treatment to standing resting normotension with beta-blocker drugs. The double product (peak mean BP times peak heart rate) at peak performance fell significantly because of decreases in systolic arterial BP and heart rate, but diastolic pressures still rose with exercise in ten of the 12 patients, in seven of them to 100 mm Hg or higher. Thus, the data show that patients with hypertension treated to resting normotension with hydrochlorothiazide and a beta-blocker still often demonstrate a significant diastolic BP rise with exercise. In such patients, a rise in diastolic BP with exercise cannot be used as evidence of coronary atherosclerosis. The measurement of the arterial BP response to exercise is probably important in the assessment of the effectiveness of individual antihypertensive regimens.