Oxidative metabolism of cannabidiol monomethyl ether (CBDM), one of the components of marihuana, was studied in the guinea pig. Cannabielsoin monomethyl ether (CBEM) was found to be formed with hepatic microsomes by gas chromatography-mass spectrometry (GC-MS). Experiments using various modifiers of enzymatic reaction suggested that, as in the case of cannabielsoin (CBE) formation from canabidiol (CBD), CBEM was formed from CBDM by the monooxygenase system including cytochrome P450. When cannabidiol dimethyl ether (CBDD), in which phenolic hydroxyl groups of CBD are masked with methyl groups, was incubated with liver microsomes and an reduced nicotinamide adenine dinucleotide phosphate-generating system, 1S,2R-epoxy-CBDD was identified by GC-MS. The epoxy metabolite was also found in the liver of a guinea pig pretreated with CBDD (100 mg/kg, intraperitoneally) 1 h before sacrifice. Rate of 1S,2R-epoxide metabolism was slower than that of 1R,2S-epoxy-CBDD under the conditions, as in the microsomal oxidation of CBDD described above. These results indicate that 1S,2R-epoxides are formed from CBD, CBDM and CBDD and that the epoxides are quickly converted to elsoin-type metabolites in the cases of CBD and CBDM.