Using the isolated perfused rat and hamster liver, the relationship between bile flow, bile acid secretion rate and bile acid biotransformation after the injection of a small, bolus dose of radioactive ursodeoxycholate or of its C₂₃ homolog, norursodeoxycholate, was examined. Ursodeoxycholate was promptly secreted into bile mostly as amino acid conjugates; <3% was secreted in unchanged form in the rat and <2% in the hamster. In contrast, norursodeoxycholate was secreted slowly, and biotransformed into glucuronide conjugates and unconjugated trihydroxy derivatives; it was also secreted in part in unchanged form. In the rat, 7% was secreted in unconjugated trihydroxy derivatives and 3% in unchanged form; in the hamster, 7% was secreted as unconjugated trihydroxy derivatives and 4% in unchanged form. The secreted bile acid species that showed the highest correlation with bile flow by far was always the unconjugated form in both rat and hamster. By multiple regression analysis, the apparent choleretic activity (microliters of induced bile flow per micromoles recovered bile acid molecules) indicated marked hypercholeresis for the unconjugated bile acid with values ranging from 100 to 300 μ/μmol. Bile flow also correlated with total bile acid recovery for ursodeoxycholate in rat and norursodeoxycholate in hamster, but in all studies the apparent choleretic activity was far lower. Other calculations indicated that most bile flow during the first 30 min was induced by secretion of the unconjugated bile acid species in all experiments, the proportion ranging from 50% to 90%. The results indicate that when a bolus of ursodeoxycholate or norursodeoxycholate is presented to the perfused rodent liver, the secretion of the unchanged bile acid appears to be responsible for most of the bile flow, probably by a cholehepatic shunting mechanism. (HEPATOLOGY 1991;13:540–550.)