We study the modifications of sodium tubular resorption, measured by lithium clearance after a single dosage of sublingual captopril, administered to 24 patients afflicted with nonfiltration after captopril produced an increase of proximal resorption of sodium, compensated by minor distal resorption, keeping a constant natriuresis. The different effects of captopril on blood pressure create 2 groups: a) patients who showed a decrease of blood pressure (n = 14), where a fall of distal resorption of sodium simultaneous with an increase of fractional sodium excretion was registered, and b) patients who did not experience changes in blood pressure nor changes after tubular function tests.