Ethinyl estradiol is a well documented, predictable cholestatic agent. The direct effects of this drug on hepatocellular uptake and efflux have been investigated. Uptake of taurocholate and ouabain, which are both actively transported, was inhibited, while uptake of cadmium, which occurs by a combination of facilitated and simple diffusion, was unaffected. The Vmax for taurocholate uptake was not altered by the presence of ethinyl estradiol, whereas the Km was increased, suggesting a reduced affinity of receptor for taurocholate. Efflux of taurocholate from pre-loaded cells remained unchanged in the presence of ethinyl estradiol. The data are not entirely consistent with an interference of Na+,K+-ATP ase by ethinyl estradiol being an important site of action. The observations may help to explain the mechanism of biliary dysfunction induced by ethinyl estradiol.