Abstract.The effect of conjugated trihydroxy bile salts, tauro‐ and glycocholate, and of deoxycholate on tissue uptake and mucosal to serosal transfer of actively transported hex‐oses and amino acids has been examined in rat small intestinein vitro.Conjugated trihydroxy bile salts and deoxycholate markedly inhibited active transport of hexoses and amino acids in the ileum of rat small intestine, whereas in the jejunum, deoxycholate alone was inhibitory. The inhibitory effect of tauro‐ and glycocholate increased with incubation time. It persisted after washing of the tissue and reincubation with hexoses in a bile salt free medium, and could be observed with only 2 × 10‐4M taurocholate. Taurocholate was able to evoke an increase of transmural potential difference (PD) in the ileum, but did not affect PD in the jejunum. Prein cubation of ileal small intestine with taurocholate depressed subsequent glucose‐induced PD‐increments. In the jejunum, however, taurocholate did not affect PD‐increments induced by D‐glucose.It is concluded that conjugated trihydroxy bile salts have to enter intestinal mucosal epithelial cells to an appreciable extent in order to affect other active, energy‐requiring transport systems in rat small intestine. Previous results showing a failure of conjugated bile salts to inhibit active transport of hexoses and amino acids are explained by the fact that only jejunal transport