The human multidrug-resistance-associated protein (MRP), a member of the adenosine triphosphate (ATP)-binding cassette transporter superfamily, is frequently overexpressed in tumor cells resistant to antineoplastic drugs. In the rat, two Mrp isoforms have been identified, Mrp and cMrp. cMrp, also called Mrp2 or cMoat (canalicular multispecific organic anion transporter), is expressed in the canalicular membrane of rat hepatocytes and mediates the excretion of glucuronate, sulfate, and glutathione conjugates into bile. We investigated the expression of cMrp and Mrp in rat hepatocytes in primary culture. Treatment with the chemical carcinogen 2-acetylaminofluorene (2-AAF), the antineoplastic drug cisplatin, and the protein-synthesis inhibitor cycloheximide led to a dose-dependent and time-dependent increase in cmrp gene expression. A 347-base pair cmrp complementary DNA (cDNA) probe served to demonstrate the induction of cmrp messenger RNA (mRNA) with 40 micromol/L 2-AAF, 5 micromol/L cisplatin, or 5 micromol/L cycloheximide. An analogous response was obtained for the increase in cMrp protein. Mrp mRNA was below the detection limit in Northern blots of RNA from liver and hepatocyte cultures, in contrast to rat testis mRNA which served as a positive control. Immunofluorescence microscopy of cultured hepatocytes was used to visualize cMrp in the plasma membrane. Treatment with 2-AAF led to a marked increase in the immunofluorescence signal confirming the cMrp-inducing potency of 2-AAF. In conclusion, the inducing effect of the compounds studied may reflect a general inducibility of hepatic cMrp by a variety of cytotoxic, carcinogenic, and chemotherapeutic agents which is likely to be of relevance for the acquisition of multidrug resistance during chemotherapy and in the process of chemical carcinogenesis in the liver.