The renal excretion of bile acids was studied in an isolated rat kidney preparation perfused with a protein free medium. Tubular reabsorption exceeded 95% for both non sulfated and sulfated bile acids at filtered loads of less than 30 nmol/min. At low filtered loads the reabsorption of taurocholate and taurochenodeoxycholate was almost complete. Efficient reabsorption of taurochenodeoxycholate was maintained over a wider range of filtered loads than for taurocholate. These observations suggest that active transport may occur. At high filtered loads saturation of reabsorption of taurocholate and taurochenodeoxycholate did not occur, which indicates that passive diffusion is involved in reabsorption. Active proximal tubular secretion of bile acids was not demonstrated in competition experiments with p aminohippurate. The fractional reabsorption of taurocholate, chenodeoxycholate 3,7 disulfate and chenodeoxycholate 7 monosulfate was decreased by the addition of taurochenodeoxycholate to the perfusate, so that their renal excretion was enhanced. This interaction between the bile acids for reabsorption may explain the different composition of bile acids in urine compared with that in plasma in cholestasis in man. Conjugated bilirubin decreased the fractional reabsorption of both taurocholate and taurochenodeoxycholate at low filtered loads (less than 30 nmol/min) but not at high filtered loads (400 nmol/min).