Administration of atrial natriuretic peptides (ANP) is followed by an immediate increase in the urinary excretion of NaCl and water. ANP modulates intrarenal segmental resistances without affecting renal blood flow to a marked extent. At higher concentrations ANP is capable of raising glomerular filtration rate which may contribute to the massive elevation of NaCl excretion seen under these circumstances. At lower concentrations of ANP NaCl excretion increases without measurable increases of GFR suggesting inhibition of transport by the peptides. Transport of fluid across proximal convoluted tubules does not appear to be inhibited by ANP to a marked extent. Atrial peptides produce a modest inhibition of loop of Henle chloride and water absorption in in situ perfused nephrons. Cl absorption along the accessible portion of the inner medullary collecting duct is not inhibited, but delivery of chloride to the earliest portions of the papillary collecting duct rises. This increase could result from a reduction in transport along the inaccessible portions of the inner medullary collecting duct, from secretion of NaCl along the outer or inner medullary collecting duct or from a change in deep nephron function.