The renal effects of the new angiotensin converting enzyme inhibitor enalapril (MK-421) and of its active metabolite MK-422, were investigated in patients with essential hypertension. Together with a reduction in blood pressure, renal blood flow increased after intravenous injection of MK-422. Glomerular filtration as well as fractional sodium excretion increased. The latter was explained by the inhibition of both proximal and distal tubular sodium reabsorption. During oral enalapril therapy renal blood flow remained elevated, whereas glomerular filtration did not differ significantly from pretreatment values. Body weight fell after 12 weeks of therapy, a possible consequence of the 'diuretic' effect of enalapril which may contribute to the antihypertensive action of the drug. In conclusion, enalapril had either no, or even beneficial, effects on renal function.