We have examined P-glycoprotein (P-gp) expression and function in cultured rat hepatocytes in response to dexamethasone (DEX), which is known to modulate various liver functions. Northern blot analyses revealed high levels of P-gp mRNAs in cultured untreated liver cells in comparison to those found in freshly isolated hepatocytes, while DEX-treated hepatocytes also displayed elevated, although weaker, P-gp levels. Similarly, Western blotting analysis indicated high levels of P-gp in liver cells maintained in the absence of DEX. The use of mdr gene-specific probes allowed us to show that DEX-modulated P-gp induction in cultured hepatocytes involved mostly, if not specifically, mdr1 gene regulation. Doxorubicin P-gp-mediated efflux analyses revealed lower intracellular doxorubicin accumulation in DEX-untreated liver cells than in DEX-treated cells, thus indicating that over-expressed P-gp was functional. These data clearly show that DEX treatment strongly modulates P-gp expression in primary rat hepatocyte cultures through a specific effect on the mdr1 gene.