1. We studied the effect of oral prazosin on blood pressure, plasma active renin and angiotensin II in 16 recumbent hypertensive patients between 09.00 and 12.00 hours.
2. In untreated patients there were no significant changes in blood pressure, plasma active renin or angiotensin II during the 3 h period.
3. After an initial dose of prazosin (2 mg) there was a significant fall in recumbent blood pressure with progressive increases in plasma active renin and angiotensin II concentrations.
4. After 3–4 weeks' treatment with oral prazosin only, recumbent blood pressure at 09.00 hours, 15 h after the last dose of prazosin, was significantly lower than before treatment, but plasma active renin and angiotensin II were not significantly different from untreated values.
5. After the usual morning dose of prazosin, blood pressure did not change, but there were significant rises in plasma active renin and angiotensin II, less marked than after the first dose of prazosin.
6. Prazosin therefore stimulates the renin—angiotensin system acutely, both after the initial dose and during long-term therapy; however, no effect on renin and angiotensin II is apparent 15 h after the last dose of prazosin.