Crude marijuana extract competed with estradiol for binding to the estrogen receptor of rat uterine cytosol. Condensed manjuana smoke also competed with estradiol for its receptor. Pure 9-tetrahydrocannabinol, however, did not interact with the estrogen receptor. Ten i9-tetrahydrocannabinol metabolites also failed to compete with estradiol for its receptor. Of several other common cannabinoids tested, only cannabidiol showed any estrogen receptor binding. This was evident only at very high concentrations of cannabidiol. Apigenin, the aglycone of a flavinoid phytoestrogen found in cannabis, displayed high affinity for the estrogen receptor. To assess the biological significance of these receptor data, estrogen activity was measured in vivo with...