The nature of transport pathway(s) for the biliary excretion of taurocholate and tauroursodeoxycholate was examined by comparing the biliary transport maximum (Tm) value for taurocholate during the infusion of taurocholate alone with that of taurocholate combined with tauroursodeoxycholate. The combined infusion of tauroursodeoxycholate resulted in an appreciable excretion of tauroursodeoxycholate while the excretion rate of taurocholate was not reduced in comparison with the Tm value of taurocholate alone. Furthermore, the Tm state of taurocholate was maintained for a much longer period with the simultaneous infusion of tauroursodeoxycholate than by the infusion of taurocholate alone. The cholestasis usually produced by the excess infusion of taurocholate was also prevented when tauroursodeoxycholate was simultaneously infused. Since plasma taurocholate concentration was not significantly different from the two rat groups, the results suggest the presence of the facilitative interaction of tauroursodeoxycholate with the taurocholate excretion.