A new beta-adrenoreceptor blocking drug (timolol) was used in two clinical studies. In the first double blind study administration of hydrochlorothiazide and timolol reduced the mean (formula: see text) blood pressure by 23 mm Hg, compared with a fall of nine mm Hg with administration of hydrochlorothiazide and a placebo. Plasma potassium (K+) levels fell with the administration of hydrochlorothiazide alone, but rose back to control values when patients were given timolol. In the second study (patient blind), Moduretic (hydrochlorothiazide, 50 mg and amiloride, 5 mg) and timolol were given once daily. The mean blood pressure fell by 20 +/- 2 mm Hg allowing hypertension to be controlled by this once-daily regimen; the control persisted throughout the day. Plasma potassium (K+) levels fell slightly after treatment with Moduretic, but the fall was not as great as with hydrochlorothiazide. Of the 65 patients who entered these studies, 35 had not been previously treated and had diastolic blood pressure between 105 mm Hg and 130 mm Hg. In 18 patients the diastolic blood pressure was reduced below 95 mm Hg after treatment with a diuretic alone, in 15 patients treatment with a diuretic and timolol (5 mg to 20 mg) reduced the diastolic blood pressure below 95 mm Hg. In two patients, the diastolic blood pressure was between 95 mm Hg and 100 mm Hg. The response achieved with a relatively simple therapeutic regimen that can be administered once daily suggests that the therapy of most hypertensive patients could be supervised by paramedical personnel.