The antihypertensive activity of terazosin, an investigational alpha 1-adrenergic-receptor blocker, and its effect on blood lipids were compared with placebo in a double-blind study. After a 3-week placebo baseline period, patients were randomized to receive terazosin (n = 22) or placebo (n = 16). The dose of terazosin was titrated over 2 weeks to a maintenance dosage of 10 mg once daily for 4 weeks. Antihypertensive efficacy was assessed: (1) at the end of the 24-hour dosing interval by comparing the average blood pressure (BP) after 3 and 4 weeks of maintenance therapy to the average BP after 2 and 3 weeks of placebo therapy, and (2) for 3 hours after drug ingestion at the final visit in comparison to the predose BP at that visit. At the end of the 24-hour dosing interval, 10 mg of terazosin reduced the mean supine systolic BP from 155.6 to 152.2 mm Hg and mean supine diastolic BP from 101.9 to 99.0 mm Hg (p less than 0.05). During the 3 hours after drug ingestion, mean supine systolic and diastolic pressures decreased maximally from 151.8 to 142.7 mm Hg (p less than 0.05) and from 99.5 to 91.0 mm Hg (p less than 0.05) respectively. No supine BP reduction differed significantly from the placebo response. During terazosin therapy there was a nonsignificant increase in mean body weight of 1.4 +/- 2.9 kg and no change in blood lipids. Thus the drug demonstrated greater antihypertensive activity 1-3 hours after ingestion than at the end of the 24-hour dosing interval.(ABSTRACT TRUNCATED AT 250 WORDS)