Expression of CYP1A1 mRNA in mouse hepatocytes in primary culture was investigated. The expression was obvious on day 3 of culture without addition of any known ligands of the aryl hydrocarbon receptor and increased with culture period. Removal of insulin from and addition of hydrogen peroxide to the medium enhanced and suppressed the expression, respectively. The CYP1A1 mRNA expression was also enhanced in the presence of anti-oxidant, t-butylhydroquinone, in the medium. Several kinds of kinase inhibitors markedly increased the CYP1A1 mRNA expression. In contrast, the inhibitory expression was prolonged in the presence of okadaic acid, a potent inhibitor of serine/threonine phosphatase PP1 and PP2. These observations suggest that there might be a repressive pathway in the regulation of CYP1A1 mRNA expression and that the presently observed expression pathway differs at several points from those previously reported, such as ligand-activated aryl hydrocarbon receptor- or omeprazole-mediated expression. Modulation of CYP1A2 mRNA expression after exposing hepatocytes to agents affecting phosphorylation pathways differed from that of CYP1A1 mRNA. This implies that regulatory pathways for CYP1A1 and CYP1A2 expression may differ.