A side chain derivative of ursodeoxycholic acid, 23-methylursodeoxycholic acid, was synthesized and the effect of i.v. infusion of the acid at different doses (0.75, 1.5, 3.0 and 6.0 mumoles per min per kg body weight over 1 hr) on bile flow, on its hepatic biotransformations and on biliary lipid secretion has been studied in bile fistula rats. The results were compared with those of ursodeoxycholic and cholic acid administered under similar conditions. 23-Methylursodeoxycholic acid is poorly secreted into bile and poorly taurine and glycine conjugated, at all infusion doses. Ursodeoxycholic acid is quantitatively recovered at low doses and recovered less at high infusion rates. Cholic acid is almost entirely recovered at all infusion doses. Ursodeoxycholic acid conjugation pattern is dependent on the dose, and glucuronidation and sulfation operate at high doses. Cholic acid is taurine conjugated at low doses; at high doses, large amounts of unconjugated bile acids are observed. Methylursodeoxycholic acid presents a delayed secretion and hypercholeresis. Ursodeoxycholic acid presents similar results at high infusion rates, possibly by reaching a high intrahepatic concentration of free form. The octanol/water partition coefficients of ursodeoxycholic acid and 23-methylursodeoxycholic acid are similar and higher than that of cholic acid. A chole-hepatic shunting of 23-methylursodeoxycholic acid may explain both the low recovery in bile and hypercholeresis and is consistent with its hydrophilicity of cholic acid, on the contrary, makes possible its high recovery in bile. The effect on biliary lipid secretion is unpredictable and affected by the dose and, in consequence, by the conjugation pattern of the bile acid.(ABSTRACT TRUNCATED AT 250 WORDS)