The acute and chronic effects of pinacidil on blood pressure (BP) and renal function were investigated in 10 patients with moderate arterial hypertension insufficiently controlled by chronic beta-blockade. Acute i.v. administration of pinacidil caused a significant fall in BP of 29.9/18.3 mm Hg and, despite beta-blockade, a concomitant rise in heart rate (HR) of 21%. Renal vascular resistance (RVR) showed a marked reduction as a consequence of the fall in BP, and a transient rise in renal plasma flow (RPF). Diuresis and renal clearance of sodium and uric acid showed a parallel fall. The excretion rates of albumin and beta 2-microglobulin were also significantly reduced. Pharmacokinetic studies indicated that glomerular filtration was responsible for elimination of the parent drug, and that proximal tubular secretion was the pathway of excretion of the main metabolite, pinacidil pyridine-N-oxide. During therapy for 4 months there was no further significant reduction in BP, despite increases in the daily dose of pinacidil. The effects on HR were less conspicuous after 4 months; renal haemodynamic parameters and body weight were not significantly changed. The initial level of RVR and the initial acute reduction in this parameter appeared to be major determinants of the long-term BP response. The drug was well tolerated apart from one patient who developed slight fluid retention. However, concomitant administration of a diuretic will probably be necessary during routine use of this therapeutic combination.