Eighteen patients ages 35-70 years (mean +/- SEM 58 +/- 2) with poorly controlled hypertension on various regimens, participated in the present studies. After a 4-week placebo-controlled lead-in period, 12 patients were randomized to terazosin treatment and 6 to placebo. They were followed in the clinic every 2 weeks for 13 weeks, where their supine (5 min) and the upright (2 min) arterial pressure and heart rate were measured. In addition, all patients had a complete laboratory evaluation at the beginning and end of the study. Depending on pressure response, the experimental drug was increased at each visit from 1.0 mg/day to 2.0, 5.0, 15.0, and 20.0 mg/day, if the supine diastolic pressure was greater than 90 mmHg. Terazosin decreased the systolic and diastolic pressure in both the supine and upright positions, and had no significant effect on heart rate. Placebo did not exert any effects on either arterial pressure or heart rate. No adverse clinical or metabolic effects were observed with the administration of either terazosin or placebo. We conclude that: Terazosin is a new effective long-acting alpha blocker given in combination with other antihypertensive drugs, and it is safe and well tolerated by the patients.