Cannabidiol hydroxy-quinone (CBDHQ) was identified as an air oxidation product of cannabidiol (CBD). The in vitro incubation of mouse hepatic microsomes with CBDHQ resulted in a decrease of cytochrome P-450 content. CBDHQ inhibited the hepatic microsomal drug-metabolizing enzymes of mice. This inhibitory effect was stronger than that of CBD. CBDHQ (150 microM) inhibited aniline hydroxylase, p-nitroanisole O-demethylase and aminopyrine N-demethylase in the microsomes by 70, 52 and 77%, respectively, whereas the same concentration of CBD caused the inhibition by 39, 30 and 26%, respectively. CBDHQ (91.5 microM) significantly decreased total heme content by 21% and free SH groups by 11% in the microsomes. The results indicate that CBDHQ, which is an oxidation product of CBD, inhibits the hepatic microsomal drug-metabolizing enzymes through the decrease of cytochrome P-450 content.