Addition of 2% dimethylsulphoxide (DMSO) to the medium has been described to retain biochemical and morphological differentiation of cultured rat hepatocytes. The maintenance of differentiated hepatocytes for a long time with stable phase I and II biotransformation capacities provides an important model for pharmacological and toxicological studies. This work has been undertaken to study the effects of DMSO on phase I and II parameters in cultured hepatocytes. For phase I cytochrome P-450 content, 7-ethoxycoumarin O-deethylase and aldrin epoxidase activities and for phase II glutathione S-transferase (GST) activity and its isoenzyme profile have been measured. In control cells cytochrome P-450 content and the enzymic activities measured declined as a function of culture time. Addition of DMSO partially prevented the loss of all parameters measured. The changes in GST activity were related to changes in its isoenzyme pattern. In control cells subunits 1 and 2 decreased, 3 and 4 increased and de novo expression of 7 was observed. When DMSO was added, the subunit profile better resembled that observed in vivo and the expression of subunit 7 was suppressed. In conclusion DMSO protects cultured hepatocytes from dedifferentiation and stabilizes the cells.