The murine Cyp2a-4 and Cyp2a-5 genes encode P450 isoforms catalysing testosterone 15 alpha-hydroxylase and coumarin 7-hydroxylase (COH) activities, respectively. Two days after the administration of a hepatotoxic dose of cerium chloride (2 mg/kg i.v.), COH activity was increased 3.2-fold in the liver of DBA/2 mice. Three and 4 days after the cerium treatment, coinciding with the occurrence of overt liver damage, there was a dramatic decrease in COH activity. The activities of testosterone 15 alpha-hydroxylase and the Cyp1a-1-mediated 7-ethoxyresorufin O-deethylase (EROD) were decreased in response to cerium. Much less pronounced changes in the enzyme activities occurred in the C57BL/6 mouse liver. Northern blot analysis showed a 21-fold increase in the hepatic Cyp2a-4/5 mRNA in the DBA/2 mice at day 2, whereas no increase occurred in the C57BL/6 mice. Also in the kidneys the increase in COH activity and in Cyp2a-4/5 mRNA was marked only in the DBA/2 mice. A polymerase chain reaction-mediated analysis method utilizing a unique PstI restriction site in the Cyp2a-5 cDNA was used to differentiate between the highly homologous Cyp2a-4 and Cyp2a-5 mRNAs. Cerium was found to increase the amount of hepatic and renal Cyp2a-4 and Cyp2a-5 mRNA only in the DBA/2 mice. These data indicate that the Cyp2a-4/5 complex is regulated in a different way in DBA/2 and C57BL/6 mice and that some association exists between the development of liver damage and COH induction.