The multidrug resistance (MDR)-associated protein, P-glycoprotein (Pgp), is expressed on the bile canalicular surface of hepatocytes, where it is thought to function in the detoxification of xenobiotics or in the transport of specific metabolites. Several studies have shown that Pgp expression in rat liver can be perturbed in vivo and in vitro; however, it is not known which of the 3 Pgp genes (class I, II, or III) are involved. In rodents, the class I and II Pgp genes have been shown to mediate MDR while the class III gene apparently does not. In this report, we have used gene-specific probes generated from the 3'-untranslated regions of the three rat Pgp genes (Deuchars et al.: Biochim. Biophys. Acta, 1130:157-165, 1992) to investigate Pgp gene expression in primary rat hepatocytes. We observed that the class II Pgp mRNA, the least abundant in the intact liver, is dramatically increased in culture over a 48 h period, while the class I Pgp showed only a modest increase in mRNA level. In contrast, the class III Pgp mRNA, which is the most abundant in the intact liver, exhibited a gradual decline. In rat liver hepatocytes, different culture conditions, as well as drugs such as cytochalasin D and colchicine, appear to affect the level of the class II Pgp gene expression. Moreover, under all these conditions, there is a strong correlation between the level of the class II Pgp and cytoskeletal (actin and tubulin) mRNAs. Thus, there may be a common mechanism regulating the expression of cytoskeletal protein genes and the class II Pgp gene. These findings have implications for our understanding of the regulation of Pgp gene expression in normal and malignant tissues.