Hepatic microsomal oxidation of 11-oxo-delta 8-tetrahydrocannabinol (11-oxo-delta 8-THC) and 9-anthraldehyde (9-AA) to the corresponding carboxylic acids was investigated using six strains of male and female mice (ddN, ddY, C57BL, DBA, C3H and ICR). No significant sex difference was observed in the activity toward 11-oxo-delta 8-THC except for ICR, whereas the activity toward 9-AA was significantly higher in female than in male of ddN, C57BL, DBA and C3H mice. The present study suggests that female specific form(s) of cytochrome P450 may be responsible at least in part for the microsomal oxidation of 9-AA, but not that of 11-oxo- delta 8-THC.