Coumarin 7-hydroxylase (COH) activity is catalyzed by the Cyp2a-5 gene product (CYP2A5 enzyme) in mice. Mouse hepatic CYP2A5 expression is often increased in conditions in which other P450 forms are repressed, e.g. after the administration of heavy metals and other toxic agents known to affect cellular heme balance. In this study, the effect of various porphyrinogenic chemicals on the expression CYP2A5 and the key enzymes in heme metabolism was studied. Administration of single doses of griseofulvin (1000 mg/kg), thioacetamide (10 mg/kg) and aminotriazole (1000 mg/kg) to DBA/2 and C57BL/6 mice produced up to 10-fold increases in hepatic COH catalytic activity. Dramatic, up to 130-fold increases in response to the inducers was observed in the amount of CYP2A5 steady-state mRNA. The mRNA contents of aminolevulinate synthase, ferrochelatase and heme oxygenase were also increased to a variable extent, possibly reflecting feed-back regulatory mechanisms. In D2 mice the CYP2A5 inducing effect of aminotriazole and thioacetamide, but not that of griseofulvin, pyrazole and phenobarbital, was abolished by exogenously administered heme arginate. In the B6 strain heme arginate treatment increased CYP2A5 expression but it did not affect the induction caused by porphyrinogenic agents. These results show that porphyrinogenic agents act as efficient inducers of CYP2A5, and suggest that regulation of the transcription of the Cyp2a-5 gene could in some instances involve heme-sensitive factors.