Because of the special physiochemical properties of ursodeoxycholate and its tauroconjugate, the effect of these two bile acids on the maximal biliary secretory rate of sulfobromophtalein was compared to that of taurocholate in the hamster. It was confirmed that taurocholate increased the maximal biliary secretory rate of the dye, while it was shown that ursodeoxycholate did not. When the bile acids were infused before sulfobromophtalein, the maximal biliary secretory rate of the dye was significantly higher under taurocholate infusion than under tauroursodeoxycholate and ursodeoxycholate infusions. Under the same conditions, the biliary bile acid output was only slightly decreased by the dye under taurocholate infusion, while it was reduced by approximately one-half under urso- and tauroursodeoxycholate infusions. These observations indicate that ursodeoxycholate is unable to increase the maximal biliary secretory rate of sulfobromophthalein (in contrast to taurocholate). It is suggested that urso- and tauroursodeoxycholate interact with sulfobromophthalein during hepatic transport and that these two bile acids may be transported by the liver by mechanisms different from those of taurocholate.