Twenty-nine subjects with mild to moderate essential hypertension completed this 13 week randomized, double-blind, placebo-controlled study comparing the antihypertensive effects of nifedipine GITS (N) (30-60 mg/day), hydrochlorothiazide (H) (25-50 mg/day) and placebo (P). Nifedipine GITS is a new formulation designed for once daily administration. N lowered blood pressure 15/10 mmHg (p less than 0.0005/p less than 0.0001), H 15/8 mmHg (p less than 0.0005/p 0.05), and P 4/0 mmHg (p = NS). All patients required the larger dose of active drug to control BP. No drug affected the heart rate. For N, but not H, there was a positive correlation between age and the magnitude in reduction of SBP (r = 0.79; p less than 0.005), but not for DPB. N drug levels did not correlate with its antihypertensive effect. Epinephrine, norepinephrine, aldosterone, and plasma renin activity were not affected by N. No patients on N experienced immediate vasodilator side effects. N is a well tolerated and effective antihypertensive drug that can be given once daily. It does not cause reflex stimulation of the sympathetic nervous system.