1. Cholic, taurocholic and dehydrocholic acids were administered I.V. to rats, rabbits and dogs (3.12-100 mg/kg). With the higher doses of cholic acid, the bile flow was increased sixfold in the dog and only 2 to 2(1/2)-fold in the rat and rabbit. The choleretic response was also maintained for a longer time in the dog (90 min) than in the rat or rabbit (20-30 min).2. Similar species differences in the choleretic response to taurocholic acid and dehydrocholic acid were observed. However, the bile flow returned to control rates more rapidly for cholic acid (10-15 min for rat and rabbit, 45 min for dog) and more slowly for dehydrocholic acid (25-40 min for rat and rabbit, 120 min for dog).3. Cholic acid is conjugated more rapidly by the rabbit and rat than dog.4. An increase in the biliary bile acid concentration was observed in all three species after the I.V. administration of the bile acid excreted by each species.5. Control bile flow was much higher in the rat (50 mul./min.kg) and the rabbit (70 mul./min.kg) than in the dog (5 mul./min.kg).6. Part of the difference in the choleretic response of the three species to the bile acids appears to be due to this difference in basal bile production.7. Administration of taurocholic acid to the rat increases the biliary bile acid concentration in the bile but the bile flow may either increase or decrease, questioning the mechanism by which bile acids increase bile flow.