Abstract Growing rats which had been allowed free access to dietswith emphasis on fat or carbohydrate for 4 weeks or to mutually exchanged diets half-way during the feeding period were examined for ileal Na +-dependent taurocholate transport, using isolated epithelial cells. High-fat (low-carbohydrate) feeding caused a statistically significant increase in the transport rate per unit tissue weight or mg protein as compared with low-fat (high-carbohydrate) feeding. A similar tendency was observed for the groups with dietary exchange. This increase reflected the raised maximal transport rate (Vmax) and was not due to the altered carrier affinity for taurocholate. The effect of experimental diabetes mellitus on the ileal membrane transport system was also investigated on the analogy of hypoinsulinemia under high-fat feeding conditions. As a result, the taurocholate transport rate in alloxan-diabetic rat ileum did not increase, or rather decreased, relative to that in normal rat ileum. Hence, insulin is unlikely to serve as a major regulator in the dietary adaptation of the bile acid transport system.