Spectral and inhibitory interactions of two methylenedioxyphenyl (MDP) compounds (dihydrosafrole (DHS) and 4,5-dichloro-1,2-methylenedioxybenzene (DCMB] and 4-n-butyl dioxolane (BD) were studied in vitro in reconstituted systems incorporating cytochromes P-450b and P-450c, purified respectively from hepatic microsomes of phenobarbital (PB)- and beta-naphthoflavone (beta NF)-treated rats. In NADPH-fortified reconstituted systems containing P-450b, DHS yielded a stable type III spectral complex with peaks at 428 and 458 nm; a complex with a single 456 nm peak was formed in systems containing cytochrome P-450c. DCMB formed unstable 456-458 nm spectral complexes with both isozymes, and BD generated an unstable complex with a single Soret peak near 428 nm with cytochrome P-450b; no spectral interaction occurred between BD and cytochrome P-450c. Carbon monoxide was formed in incubations of DCMB with both isozymes but was not observed with either DHS or BD. Marked selectivity was observed in the ability of the test compounds to inhibit selected mono-oxygenase reactions in the reconstituted systems. Thus, while DHS was an effective inhibitor of cytochrome P-450b-mediated ethoxycoumarin O-deethylase (ECD), it failed to inhibit aldrin epoxidase (AE) in the same system; DCMB and BD inhibited both of these reactions. In reconstituted systems incorporating cytochrome P-450c, DHS and DCMB, but not BD, were effective inhibitors of ethoxyresorufin O-deethylase (ERD) activity but none of the compounds showed any inhibitory activity towards aryl hydrocarbon (benzo[alpha]pyrene)hydrolase (AHH) activity. The results indicate that metabolite complex formation with cytochrome P-450 is not the sole criterion for inhibition of mono-oxygenase activity by MDP and related compounds, and that in some cases type I competitive interactions at the substrate binding sites may be the primary contributing factor.