Urapidil is a new antihypertensive agent involving both a peripheral and a central mode of action. To evaluate the acute effects of this drug on renal vascular tone and on pressor systems a randomized placebo-controlled crossover study was conducted in 10 patients with uncomplicated essential hypertension. Each subject received, on 2 separate days 1 week apart, an intravenous injection of either placebo or urapidil (25 or, if necessary, 50 mg). Before and after this injection blood pressure and heart rate (Dinamap), renal plasma flow (125I-hippuran), active plasma renin concentration, angiotensin II, aldosterone and catecholamines in plasma were measured. The results show that urapidil, when compared with placebo, greatly reduced blood pressure, while increasing heart rate, renal blood flow, and noradrenaline and adrenaline levels. However, dopamine levels were suppressed. Whereas renin and angiotensin II were only mildly stimulated, aldosterone levels increased significantly. It is concluded that urapidil, given intravenously, has an immediate blood pressure-lowering effect associated with a decrease in renal vascular tone and an increase in renal perfusion. Consequently, both the sympathetic and renin-angiotensin systems are stimulated, although the latter only to a mild degree. The increase in aldosterone may be partially related to the decrease in dopamine levels.