All cytochrome P450 (CYP) enzymes contain heme as a prosthetic group. In contrast to other CYP enzymes, murine CYP 2 A 5 is upregulated in vivo by several agents that disturb cellular heme balance. To test the hypothesis that porphyrinogenic agents have the common feature of being able to increase CYP 2 A 5 expression, mouse liver primary hepatocytes were exposed to various porphyrinogenic chemicals and changes in CYP 2 A 5 catalytic activity and levels of mRNA were monitored. Phenobarbital increased hepatic CYP 2 A 5-mediated coumarin 7-hydroxylase (COH) activity (13.2-fold) and the amount of CYP 2 A 5 steady-state mRNA (10.6-fold). Hepatocyte COH activity was increased also by the ferrochelatase inhibitor griseofulvin and the protoporphyrinogen oxidase inhibitor acifluorfen (about 9-fold induction). Of these inducers, only phenobarbital affected CYP 1 A 12 and CYP 2 B 10 expression. In contrast, many other porphyrinogenic agents such as cobalt, 2,2,4-trimethyl-1,2-dihydroquinoline (TMDQ), 1-[4-(3-acetyl-2,4,6-trimethylphenyl)-2,6-cyclohexanedionyl]-O-eth yl propionaldehyde oxime (ATMP), aminotriazole, and thioacetamide either decreased or had no effect on CYP 2 A 5. The increases in COH activity and CYP 2 A 5 mRNA were unaffected by combined treatment with the inducers and heme arginate, suggesting that heme is not a regulator of CYP 2 A 5 induction. Treatment with actinomycin D totally abolished both constitutive CYP 2 A 5 expression and its inducibility, suggesting that a transcriptional component is involved. These data suggest that, in mouse primary hepatocytes, CYP 2 A 5 induction is not a universal response to disturbed cellular heme biosynthesis.