Cannabidiol induced a carbon monoxide-like complex with mouse hepatic microsomal cytochrome P-450 during NADPH-dependent metabolism in vitro on a spectral basis. The reduction by dithionite was required for the maximal development of a spectrum. The complex showed a peak at 450 nm which shifted to 419 or 423 nm, respectively, by further addition of hemoglobin or myoglobin. Cannabidiol-induced complex formation required molecular oxygen, and was decreased by the addition of inhibitors of cytochrome P-450-dependent monoxygenase. Pretreatment of mice with phenobarbital (80 or 100 mg/kg, i.p. for 3 days) but not 3-methylcholanthrene (80 mg/kg, i.p.) increased the complex formation. In contrast, pretreatment with cobaltous chloride (40 mg/kg, i.p. for 3 days) decreased the complex formation. 8,9-Dihydro- and 1,2,8,9-tetrahydrocannabidiols also induced the same spectrum as that of above complex, whereas cannabidiol monomethyl- and dimethylethers reduced this ability. In addition, both cannabidivarin and cannabigerol induced the complex formation, although delta 9-tetrahydrocannabinol, cannabinol and cannabielsoin did not. Olivetol but not d-limonene induced the spectrum of the complex to some extent. These results indicate that cannabidiol induces a carbon monoxide-like complex with cytochrome P-450 during hepatic microsomal metabolism, and suggest that phenobarbital-inducible cytochrome P-450s mediate at least one of the metabolic steps of CBD to form the complex, as well as the importance of the resorcinol moiety of CBD for the complex formation.