Data are presented on 22 hypertensive patients in an 18-week, double-blind comparison of trimazosin and methyldopa during which treatment with polythiazide was continued. Maximum daily doses of trimazosin and methyldopa were 800 and 2000 mg, respectively. Eight of nine patients receiving trimazosin and 8 of 12 receiving methyldopa had excellent or good overall responses. (One trimazosin patient was not evaluated for overall response.) Quantitative criteria of blood pressure response indicated that trimazosin was as effective as methyldopa. There were no clinically significant changes in results of Holter monitor recordings, ECGs, chest x-ray films, cardiopulmonary tests, or ophthalmoscopy. There were no abnormalities in laboratory tests of trimazosin patients. One patient receiving methyldopa had a positive Coombs' test. Significant side effects developed in two methyldopa patients-syncope in one patient and postural hypotension in the other. No significant side effects occurred in the patients taking trimazosin.