The effect of the selective alpha 1-antagonist terazosin on serum lipoproteins and certain blood pressure-regulating factors was assessed in 15 patients with essential hypertension. Terazosin given during 8 weeks reduced arterial pressure (from 153/103 +/- 3/2 (SE) to 143/96 +/- 5/2 mm Hg; P less than 0.02) but did not modify body weight, heart rate, blood volume, plasma renin activity, aldosterone and catecholamine levels, or serum cholesterol, triglycerides, and their lipoprotein fractions. In nine of the patients, blood pressure control was not achieved with terazosin monotherapy and the diuretic methyclothiazide, 2.5 mg, was added. After 8 weeks of combined treatment, blood pressure decreased further (P less than 0.05); serum lipids and lipoprotein fractions did not change as compared with placebo or terazosin conditions. These findings indicate that terazosin in monotherapy does not unfavorably influence lipid metabolism.