To further assess bile acid transport by the developing rat liver, we compared the rate of efflux of taurocholate from hepatocytes isolated from suckling and mature rat livers. Cell content of taurocholate (nmol/mg cell protein), after preloading with [14C]-radiolabeled plus cold bile acid (5-100 microM) was similar in both groups. Total taurocholate efflux, estimated by the decrease in cell taurocholate content, was unexpectedly greater from suckling rat hepatocytes. There was a higher bile acid efflux rate over time and a lower final cell content. Efflux from suckling rat hepatocytes was increased after preloading in incubation concentrations of taurocholate which were above the physiologic range of portal blood concentrations. Inasmuch as the bile acid binding protein content is known to be reduced in the cytoplasm of developing rat liver, intracellular taurocholate may exist largely as free ligand and thus be more readily diffusable. We speculate that the in vivo correlation of enhanced efflux is back diffusion of bile acid from the cell into the sinusoid. The effect could, in part, account for the known absence of a lobular gradient for bile acid uptake in suckling rats and, therefore, contribute to the inefficient hepatic transport of bile acid observed in developing rat liver.