P-glycoprotein, an energy-dependent plasma membrane drug-efflux pump capable of reducing the intracellular concentration of a variety of hydrophobic xenobiotics, is encoded by mdr1, a member of the multidrug-resistant (mdr) gene family. The physiological function of this protein is unknown. Because of its location on the bile canalicular domain of the hepatocyte, we and others have hypothesized that P-glycoprotein may have a physiological role as a biliary transporter of xenobiotics and endobiotics and that its expression may therefore be altered in cholestasis. Both obstructive and alpha-naphthylisothiocyanate-induced cholestasis increased mdr1a and 1b gene expression in rat liver. Hepatic P-glycoprotein levels were also increased, and the protein remained localized at the biliary hepatocyte domain. Induction of mdr1a and mdr1b gene expression in rat liver was accomplished by means of increased transcription. alpha-Naphthylisothiocyanate-induced cholestasis in cynomolgus monkeys increased hepatic expression of both the mdr1 and 2 genes. To investigate the possible role of P-glycoprotein as a biliary efflux transporter, biliary excretion of vinblastine, a representative substrate of P-glycoprotein, was studied in rats. Increased hepatic mdr messenger RNA and P-glycoprotein levels, mediated by the xenobiotic inducer 2-acetylaminofluorene, resulted in a significant increase in biliary excretion of vinblastine, which was antagonized by the P-glycoprotein inhibitor verapamil. These findings suggest that P-glycoprotein functions as a biliary efflux pump for xenobiotics and, possibly, for unidentified physiological inducers that may mediate increased transcription of the mdr gene observed during cholestasis.