A double-blind placebo crossover trial (2 periods, each of 6 wk) was carried out in 12 patients. In the first period the patients were randomly allocated to either their individual established dose of prazosin or to the same number of placebo tablets; treatment was reversed after 6 wk. Blood pressure was higher by 17/8 mm Hg in the lying posture and by 24/14 mm Hg in the standing posture during placebo than during prazosin treatment. Standing pulse rate and body weight were higher and plasma renin activity lower during prazosin treatment. Postural hypotension occurring 1 to 2 hr after the first few doses was noted in one third of the patients when they resumed prazosin treatment after the placebo course.