Primary cultures of adult rat hepatocytes would provide a suitable system for the study of hepatic drug metabolism/toxicity provided that the drug-metabolizing enzymes could be maintained at levels approaching those seen in vivo. It has been reported that culture of adult rat hepatocytes in the presence of 2% (v/v) dimethyl sulphoxide (DMSO) allowed partial maintenance of total cytochrome P450 content. However, the levels of the individual isozymes were not determined. Culture of rat hepatocytes in the presence of DMSO did maintain the total cytochrome P450 content at 65% of the fresh cell value after 7 days of culture. This was accompanied by high cytochrome P420 levels suggesting that the solvent was stimulating de novo synthesis rather than maintaining existing enzyme. In the presence of DMSO the level of ethoxyresorufin-O-deethylase (EROD) rose 4-fold in culture, whilst that of pentoxyresorufin-O-dealkylase fell rapidly indicating that the isozyme pattern was altered significantly. The increases in total cytochrome P450 content and EROD activity were prevented by cycloheximide confirming that de novo protein synthesis was occurring. Haem oxygenase activity was significantly reduced and aminolaevulinic acid synthetase was significantly increased in the presence of solvent, suggesting increased haem availability for incorporation into cytochrome P450. However increased haem availability is insufficient in explaining the isozyme specificity of cytochrome P450 induction. Hepatocytes cultured in the presence of 2% (v/v) DMSO were markedly more responsive to 1,2-benzanthracene, with EROD increasing approximately 40-fold.