The purpose of this study was to evaluate the natriuretic effect and renal haemodynamic changes induced by enalapril in patients with essential hypertension. In a group of 11 patients with mild to moderate hypertension with normal renal function, and on a controlled sodium intake (80 mmol/day), a decrease in systolic and diastolic blood pressure was observed (p less than 0.001) after 16 weeks of enalapril treatment (20 mg/day), without a change in heart rate. An increase in plasma renin activity (p less than 0.05) without changes in serum aldosterone, and a decrease in exchangeable sodium (p less than 0.001) were present at the end of the treatment period. In 10 hypertensive patients also taking a dietary sodium of 80 mmol/day, the renal haemodynamics, humoral changes, and urinary sodium excretion were measured during 4 days of enalapril treatment (20 mg/day). There was an increase in urinary sodium excretion on the 3rd and 4th days of treatment (p less than 0.01). The effective renal plasma flow and fractional sodium excretion increased 72 hours after the beginning of treatment (p less than 0.01); the glomerular filtration rate did not change, and filtration fraction decreased at 72 hours. Mean blood pressure fell 2 hours after the first dose (p less than 0.01), and the maximum drop in intrarenal vascular resistance occurred after 72 hours of treatment (p less than 0.01). Plasma renin activity increased (p less than 0.05) and serum aldosterone decreased (p less than 0.01) 2 hours after the first dose. Thereafter, serum aldosterone increased progressively until it reached values similar to those with placebo at 48 and 72 hours of treatment. Urinary kallikrein fell during the 2nd and 3rd day of treatment (p less than 0.01). It was concluded that the decrease in exchangeable sodium was due to a natriuretic effect of enalapril. This effect presumably results from renal haemodynamic changes due to the reduction of angiotensin II. Other mechanisms, such as the reduction of aldosterone and accumulation of kinins, could be contributory factors.