Acute renal failure (R.F.) rabbits induced by the muscular injection (i.m.) of HgCl2 (10 mg/kg) produced a significant increase of blood urea nitrogen concentration (BUN) and plasma creatinine concentration (Cre.) after a few days. No significant change was observed for free fatty acid (FFA) level. The pharmacokinetics of furosemide in acute renal failure rabbit was also studied. The apparent volume of tissue compartment (V2) slightly increased. The elimination rate constant (k10) from the central compartment significantly decreased, and mutually correlated with BUN or Cre. In vitro equilibrium dialysis measurement showed that the protein binding of furosemide (FR) significantly decreased in the plasma obtained from R.F. rabbits as well as that from patients with acute renal failure. Calculations made according to Sandberg-Rosenthal''s formula revealed that the maximum binding concentrations (nP) were not changed but binding constant (K) decreased in the R.F. rabbit. The presence of endogenous substances that inhibit the plasma protein-FR binding competitively may be suggested from the above result.