We have analyzed the expression of the DNA repair genes O6-methylguanine-DNA methyltransferase (MGMT) and N-methylpurine-DNA glycosylase (MPG) at RNA and protein activity level in primary rat hepatocytes in vitro and various rat hepatoma cell lines exhibiting different status of differentiation. The basal level of MGMT mRNA and activity correlated well with the degree of differentiation, as measured by tyrosine aminotransferase (TAT) mRNA expression. Induction of MGMT mRNA and protein activity by X-ray and N-methyl-N'-nitro-N-nitrosoguanidine (MNNG) treatment was most pronounced in the well-differentiated hepatocytes and in various differentiated hepatoma cell lines (up to 6-fold). There was virtually no induction in H5 hepatoma cells which exhibited the lowest degree of differentiation and expressed only low amounts of MGMT. For the other hepatoma cell lines tested, MGMT induction did not clearly correlate with TAT expression. Thus, Fao cells which exhibited a high degree of differentiation responded only very weakly with respect to induction. The results indicate that the basal level of MGMT mRNA expression is dependent on liver-specific regulatory factors, whereas the inducibility is a more complex phenomenon not solely dependent on them. Contrary to MGMT, MPG was constitutively expressed at relatively high amounts in all cell lines tested and no correlation was apparent with the degree of differentiation. MPG activity was significantly induced by mutagen treatment only in H4IIE cells. The tumor promoter phenobarbital induced MGMT, but not MPG mRNA in hepatocytes. The results indicate that MGMT and MPG are not co-regulated. Hepatoma cells with low MGMT level were most sensitive to MNNG-induced cytotoxicity. On the other hand, no correlation was apparent between MPG activity and sensitivity of the cell lines to methylating agents indicating that the MPG level is not predictive for alkylating drug resistance.