The effect of treatment with vanadate on the metabolism of bile acids was studied in normal and diabetic rats. In the normal rats, the composition of biliary bile acids was not changed by drinking an aqueous solution of 0.2 g/l NaVO3-5 g/l NaCl ad libitum for two weeks. By contrast, the increased proportion of cholic acid, accounting for 88% of the total biliary bile acids, in the diabetic rats decreased to 46% by the treatment with vanadate without any elevation of serum insulin level. These results indicate that vanadate with an insulin-like effect on glucose metabolism in diabetic rats has such an effect also on bile acid metabolism in an insulin-deficient state.