Reserpine in different doses was assigned in random, double-blind fashion to 329 patients with mild to moderate hypertension who had not achieved normotension with chlorthalidone therapy alone. The additional reduction of BP averaged 11.0/10.4 mm Hg with chlorthalidone, 50 mg, plus reserpine, 0.25 mg (C 50+R 0.25); 9.5/9.4 mm Hg with C 50+R 0.125; 6.4/8.5 mm Hg with C 50+R 0.05; and 9.9/9.6 mm Hg with C 25+R 0.125. The percentage of patients in whom control was achieved at diastolic BP less than 90 mm Hg and at least 5 mm Hg below baseline with either chorthalidone alone or with reserpine added was 65% with C 50+R 0.25, 69% with C 50+R 0.125, 58% with C 50+R 0.05, and 56% with C 25+R 0.125. Side effects of lethargy and impotence noted by patients with the 0.05-mg dose of reserpine were only one third of those noted with the 0.25-mg dose, although the incidence of other side effects did not differ. These results indicate that hypertension in many persons can be controlled by less than customary doses of reserpine in combination with a diuretic.