Renal hemodynamic and natriuretic effects of bumetanide and furosemide were studied by clearance and micropuncture methods in cats. Furosemide (0.04-5.0 mg/kg i.v.) effects increased with increasing doses up to a maximal fractional Na (FENa) and water (V/GFR [urine flow rate/glomerular filtration rate]) excretion of 23%. GFR and superficial single nephron GFR remained constant. Renal blood flow (RBF) increased up to 45% over control. A preferential increase in outer cortical RBF occurred at 5.0 mg/kg. Bumetanide (0.001-0.14 mg/kg i.v.) raised FENa and V/GFR maximally by 26%. Only at a supramaximal dose (0.69 mg/kg) did bumetanide raise RBF and depress filtration fraction. Probenecid (50 mg/kg i.v.) failed to block the natriuretic effect of bumetanide (0.025 .mu.g/kg .cntdot. min) when infused into renal artery but abolished the natriuretic action of furosemide (1.0 .mu.g/kg .cntdot. min). Probenecid failed to displace the dose-response curve for bumetanide.