Isolated canine kidneys were perfused at a constant systolic pressure of 140 mm Hg with autologous blood. Infusion of maximally effective amounts of furosemide (F) increased the fractional excretion of Cl (FEcl) from 0.03 to 0.18. Parallel changes in FENa and FEH2O were observed and, at maximal response, urine/plasma osmolality was 0.91. Ethacrynic acid (EA) was then added to the infusion fluid and produced a further increase in FECl to 0.28. Urine/plasma osmolality returned to unity. When the order of administration of the two diuretics was reversed, the response to EA alone was greater than after F (from FECl 0.05 control to 0.30 after EA) and the urine became isosmotic (urine/plasma osmolality = 1.0). After F was added, FECl increased further to 0.39. K secretion induced by F or by EA was demonstrable in all experiments. These data indicate that: 1) EA is more effective than F; 2) both drugs can completely inhibit Cl- transport in the thick ascending limb; 3) EA is more effective than F in blocking NaCl transport at sites distal to the loop of Henle; and 4) the proximal tubule is a site of action of F and possibly of EA. Whether the two drugs act entirely on membranal transport systems or, in part, by modifying the production of release of intrarenal substances and/or by diverting blood from one region to another cannot be ascertained from this work.