Changes in cytochromes P-450, aminopyrine N-demethylase (APDM), aromatic hydrocarbon (benzo[a]pyrene) hydroxylase (AHH), and type III spectral complex formation were measured in hepatic microsomes of control, phenobarbital (PB)-, and beta-naphthoflavone (beta NF)-induced rats after a single dose of dihydrosafrole (4-n-propyl-1,2-methylenedioxybenzene, DHS). Time profiles of changes in these microsomal parameters were complex and showed that APDM activities and cytochrome P-450 levels decreased immediately after treatment and were associated with concurrent increases in the intensity of the type III methylenedioxyphenyl (MDP) metabolite/cytochrome P-450 spectral complex. In noninduced rats, both APDM activity and cytochrome P-450 levels returned to control levels between 12 and 24 hr after treatment with DHS and subsequently increased above control levels. In PB- and beta NF-induced animals, the inhibitory phases were more prolonged and activity never returned to levels higher than the corresponding controls. AHH activity was increased substantially (two- to three-fold) in all cases after DHS administration. Although displacement of the MDP metabolite/cytochrome P-450 complex with 2-methylbenzimidazole generally led to a marked restoration of cytochrome P-450 levels and partially reversed the inhibition of APDM, it had little or no effect on AHH activities.