In a double-blind study of two populations-one in Birmingham, Alabama, the other in Encinitas, California-a total of 32 hypertensive patients whose blood pressure was not controlled by thiazides alone were evaluated for their response to trimazosin. Patients showing a persistent diastolic blood pressure above 90 mm Hg while receiving 2 mg polythiazide once a day (at Birmingham) or 50 mg hydrochlorothiazide twice a day (at Encinitas) during a baseline 4-week period were randomly assigned to trimazosin and placebo groups. At both clinics, patients had a greater blood pressure response to trimazosin than to placebo in both supine and standing positions. When the data were pooled, the change in supine blood pressure (systolic/diastolic) between the average baseline values and the double-blind period was more than twofold greater for the trimazosin group, a decrease of 12.3/10.9 mm Hg (p less than 0.001). A lesser decrease of 5.4/5.4 mm Hg occurred in the placebo group, significant only for the diastolic pressure (p less than 0.01). Comparative differences between the trimazosin and the placebo groups were more marked for the standing blood pressures, -11.7/-10.0 mm Hg (p less than 0.0001) vs no significant change, +2.3/-2.8 mm Hg, respectively. Little change occurred in the patients' heart rates in either position in either treatment group. Adverse reactions were not clinically important and occurred in about half of the patients in the trimazosin and placebo groups. Therapy was not discontinued in either group because of adverse reactions.(ABSTRACT TRUNCATED AT 250 WORDS)