The progesterone 17α-hydroxylase activity, which is one of the steroidogenic enzymes in rat testis microsomes, was significantly inhibited by crude marijuana extracts from Δ9-tetrahydrocannabinolic acid (THCA)- and cannabidiolic acid (CBDA)-strains. Δ9-Tetrahydrocannabinol, cannabidiol and cannabinol also inhibited the enzymatic activitiy with relatively higher concentration (100–1000 μM). Testosterone 6β- and 16α-hydroxylase activities together with androstenedione formation from testosterone in rat liver microsomes were also significantly inhibited by the crude marijuana extracts and the cannabinoids. Crude marijuana extracts (1 and 10 μg/ml) of THCA strain stimulated the proliferation of MCF-7 cells, although the purified cannabinoids (THC, CBD and CBN) did not show significant effects, such as the extract at the concentration of 0.01–1000 nM. These results indicate that there are some metabolic interactions between cannabinoid and steroid metabolism and that the constituents showing estrogen-like activity exist in marijuana.