In a multicenter, double-blind, placebo-controlled study, the efficacy and safety of terazosin and prazosin were compared in patients with mild to moderate essential hypertension. Patients were randomly assigned to one of three treatment groups: terazosin administered once daily, prazosin administered twice daily, or placebo. After a two-week placebo lead-in period, each patient received increasing doses of terazosin or prazosin at two-week intervals until the supine diastolic blood pressure was decreased by 7 mm Hg or more compared with baseline, or until the maximum dose was reached. Patients in the placebo treatment group were similarly "titrated." A patient continued taking the selected dose for the remainder of the 14-week study. A total of 174 patients entered the active treatment period of the study; 155 were included in the efficacy analysis. Once-daily administration of terazosin resulted in significant (p less than or equal to 0.05) decreases in supine and standing diastolic blood pressure (-7.6 mm Hg and -8.3 mm Hg, respectively) when compared with placebo (-4.1 mm Hg and -2.2 mm Hg). The mean decrease in supine diastolic blood pressure resulting from twice-daily administration of prazosin (-4.9 mm Hg) was not significant when compared with placebo, whereas the decrease in mean standing diastolic blood pressure observed in the prazosin-treated group (-6.1 mm Hg) was significantly greater than that observed with placebo. Similar numbers of patients in all three groups reported adverse experiences of a subjective nature. In conclusion, once-daily administration of terazosin appears to be as safe and at least as effective as twice-daily administration of prazosin for the treatment of mild to moderate hypertension.