A multicenter, double-blind study compared the antihypertensive efficacy and safety of doxazosin and terazosin as once-daily therapy. Doxazosin, a potent antihypertensive agent, selectively inhibits alpha 1 adrenoceptors. Its pharmacokinetic profile, including gradual onset of action, long plasma elimination half-life and long duration of action, permits once-daily dosing. Terazosin, a structural analog of prazosin, also inhibits alpha 1 adrenoceptors and is recommended as once or twice-daily therapy. Nineteen (73%) of 26 patients randomly assigned to receive doxazosin were therapeutic successes; 17 (65%) achieved normalized blood pressure (defined as blood pressure less than or equal to 90 mm Hg). The mean final daily dosage in patients classified as therapeutic successes was 2.4 mg. Eighteen (64%) of 28 terazosin-treated patients were considered therapeutic successes; 16 (57%) achieved normalized blood pressure. The mean final daily dosage in patients classified as therapeutic successes was 5.6 mg. Treatment-related side effects were observed in 30% of doxazosin-treated and 39% of terazosin-treated patients. Most side effects observed in either treatment group were mild or moderate and either disappeared or were tolerated with continued therapy. Doxazosin is an effective, well-tolerated, once-daily antihypertensive agent; it is comparable with terazosin but at a lower daily dosage.