The effects of alpha methyldopa and hydrochlorothiazide therapy on exercise induced changes in arterial pressure, heart rate, rate-pressure product and total duration of exercise were evaluated with graded treadmill exercise. Although both agents were equally effective in reducing resting arterial pressure, alpha methyldopa appeared to provide slower build-up, a lower ceiling and more rapid recovery of arterial pressure in response to exercise stress than did hydrochlorothizide. Alpha methyldopa significantly reduced the degree of rise of the rate-pressure product (an index of myocardial oxygen consumption) during the exercise and recovery phases, whereas hydrochlorothiazide failed to do so. The total duration of exercise (an index of work capacity) was unchanged with either hydrochlorothiazide or alpha methyldopa. Therapy with alpha methyldopa presumably reduced myocardial oxygen demand without reducing work capacity.