The effect of captopril on renal plasma flow (RPF), glomerular filtration rate (GFR), plasma concentrations of renin, angiotensin II (Ang II), aldosterone (Aldo) and arginine vasopressin (AVP), serum osmolality (Sosm), free water clearance (CLH2O), fractional urinary excretion of electrolytes and blood pressure (BP), was examined in 10 patients with moderate essential hypertension. Placebo plus furosemide was given for 6 weeks, followed by a 6-week period on captopril and furosemide. The renin-angiotensin-aldosterone system was partly blocked by captopril, resulting in a 18% fall in BP. GFR was reduced by 8% during captopril treatment, whereas RPF remained unchanged. Water loading-induced suppression of Ang II and AVP was inhibited by captopril therapy. Without affecting BP in the placebo period, water loading almost abolished the captopril-induced reduction in BP. It is concluded that non-acute captopril treatment of moderates essential hypertension reduces GFR slightly, possibly by producing dilatation of efferent arterioles. The antipressor effect of captopril is lessened during volume expansion, which may be secondary to reduced activity of the renin-angiotensin-aldosterone and osmoregulatory systems.