The pharmacokinetics and the biliary and urinary excretions following intravenous administration of furosemide (5 mg/kg) were investigated in the anesthetized dogs with normal and experimentally reduced renal function. After the administration, furosemide caused diuretic and choleretic response, and was excreted into urine and bile at almost similar rate to plasma concentration decay in normal dogs. Half maximum diuretic response was obtained at 1.5 micrograms/ml of plasma concentration and 100 micrograms/min of urinary excretion rate of furosemide. Acute renal failure was produced in dogs by the intravenous administration of mercuric chloride (HgCl2, 2 mg/kg). In HgCl2-treated dogs, the prolongation of half life (T1/2 beta) and the decrease in plasma clearance were noted with the decreased diuretic response. These changes in parameters appeared to be associated with the decrease in excretion of furosemide into the urine, but not into the bile. Plasma level-diuretic response relationship was extensively shifted to the right in HgCl2-treated dogs, while urinary dose-response relationship did not change significantly between two groups. These results suggest that the decreased response to furosemide in HgCl2-treated dogs seems to be due to the decreased renal clearance rather than to the subsensitivity to furosemide on the site of action.