To test the influence of an inhibitor of angiotensin-converting enzyme, teprotide (SQ 20881), we administered it to seven patients with essential hypertension and normal renal function and nine with an unequivocal reduction in creatinine clearance, caused by bilateral renal-artery stenosis in two and by essential hypertension in seven. Despite the fall in blood pressure (112.7 +/- 4.5 to 100.3 +/- 3.9 mm Hg, mean +/- S.E.M., P less than 0.01), there were prompt increases in both creatinine clearance (95.9 +/- 10.5 to 109.9 +/- 9.5 ml per minute per 1.73 m2 of body-surface area, P less than 0.01) and sodium excretion (17.0 +/- 5.9 to 31.7 +/- 7.2 mumol per minute, P less than 0.01) in patients with essential hypertension. The increase in glomerular filtration rate was most striking, averaging 33 per cent (66.0 +/- 10.3 to 88.0 +/- 9.2 ml per minute per 1.73 m2, P less than 0.001) in patients in whom an initial reduction was evident and hypertension was more severe. These observations suggest that a functional element, perhaps involving angiotensin-mediated renal vasoconstriction, frequently has a role in the reduction in glomerular filtration rate that occurs in essential hypertension. This class of agent may improve renal excretory function as it controls hypertension.