The direct-acting vasodilator, endralazine, in combination with a beta-adrenoceptor blocker significantly reduced the blood pressures of normotensive volunteers and of patients with essential hypertension. The mean terminal elimination half-life for endralazine of 136 min in hypertensive patients did not differ significantly from the 155 min in normotensive subjects. In normal subjects the mean oral bioavailability for endralazine was 75% and the mean clearance was 780 ml/min. There were no significant pharmacokinetic differences between fast and slow acetylators. The administration of endralazine to steady state in the hypertensive patients was associated with an increase in terminal elimination half-life and a decrease in the rate of absorption. However, there was no accumulation of endralazine with chronic dosing.