We compared hydrochlorothiazide and propranolol hydrochloride for monotherapy of hypertension by a double-blind study of 683 men who were titrated to less than 90 mm Hg diastolic BP or to 640 mg of propranolol or 200 mg of hydrochlorothiazide. Propranolol reduced systolic BP from 146.0 +/- 14.4 (SD) to 134.8 +/- 16.3 mm Hg and diastolic BP from 101.6 +/- 4.6 to 90.5 +/- 7.5 mm Hg. Hydrochlorothiazide lowered both systolic BP more effectively from 146.5 +/- 15.8 to 128.8 +/- 12.2 mm Hg and diastolic BP from 101.3 +/- 4.5 to 89.4 +/- 6.5 mm Hg. In blacks, hydrochlorothiazide lowered systolic BP 20.3 +/- 14.3 mm Hg v 8.2 +/- 12.2 mm Hg for propranolol; hydrochlorothiazide reduced diastolic BP 13.0 +/- 7.0 mm Hg v 9.5 +/- 7.0 for propranolol. In whites, the systolic BP reductions were 15.3 +/- 12.0 mm Hg for hydrochlorothiazide v 13.2 +/- 13.1 mmn Hg for propranolol; diastolic BPs were 10.9 +/- 5.7 mm Hg for hydrochlorothiazide and 12.6 +/- 6.6 mm Hg for propranolol. In blacks treated with hydrochlorothiazide, 71.3% achieved diastolic BP of less than 90 mm Hg, v 53.5% with propranolol. There was no racial difference in dose response to propranolol, but blacks required much less hydrochlorothiazide to achieve control. We conclude that in this short-term study propranolol was as efficacious as hydrochlorothiazide in whites, but the latter was more effective than propranolol in blacks.