Regulation of cytochrome P-450 isozymes 1a-1, 1a-2, and 2b-10 by methylenedioxyphenyl compounds was studied by measuring levels of mRNA, protein, and enzyme activity in hepatic tissue from C57BL/6 (Ah+) and DBA/2 (Ah-) mice dosed with isosafrole (ISO) or piperonyl butoxide (PBO). Increases in 1a-2 and 2b-10 protein were observed for ISO and PBO in both strains of mice, suggesting an Ah receptor-independent mechanism for induction of these isozymes; 1a-1 induction, however, was seen only in C57 mice. Piperonyl butoxide was the more potent inducing agent in both strains. In C57 mice treated with five dose levels of PBO, induction of 1a-1 mRNA, protein, and enzyme activity were seen at doses equal to or greater than 104 mg/kg, but were not detected at lower doses. With isosafrole, induction of 1a-1 mRNA was observed only at the highest dose tested (400 mg/kg); however, neither 1a-1 protein nor increased enzymatic activity was seen at this dose. Dose-response studies showed maximum inducible levels for 1a-2 and 2b-10 protein, beyond which the mRNAs continued to increase while the protein levels remained constant.