The antihypertensive effects of prazosin in relation to its kinetics were studied after single doses (intravenous and oral, 0.5 mg) and during increasing multiple doses (0.5 to 5 mg three times a day). There was a fall in systolic and diastolic blood pressures of 10% to 14%, which was greater in the standing than in the sitting position. Prazosin plasma concentrations correlated with dose (P less than 0.001). After intravenous prazosin the fall in systolic and diastolic blood pressure and prazosin plasma concentration correlated (P less than 0.01) during the beta-elimination phase in all patients. In only five of eight patients, however, did mean plasma concentration and antihypertensive effect during continuous treatment with different doses correlate. The maximal fall in systolic blood pressure correlated (P less than 0.01) with that after the first oral steady-state dose (0.5 mg three times daily), which indicates limited possibility of early identification of prazosin responders. There were no signs of overshoot of blood pressure when prazosin was withdrawn for a week. On rechallenge with a single oral dose of 2.5 mg prazosin there were no signs of enhanced hypotensive effect.