1. The in vitro and in vivo effects of cannabidiol on mouse and human liver microsomal metabolism of the immunosuppressive drug cyclosporine and the psychoactive compound tetrahydrocannabinol have been examined. 2. Preincubation of mouse or human liver microsomes with cannabidiol decreased the formation of all detectable cyclosporine metabolites by 73-89%. 3. In vivo cannabidiol treatment of mouse similarly decreased the formation of all detectable cyclosporine metabolites by 60-86%. 4. Preincubation of human liver microsomes with cannabidiol selectively decreased the formation of tetrahydrocannabinol metabolites catalyzed by cytochrome P4503A by 60% but had no effect on P4502C9-catalyzed metabolites. 5. Cannabidiol has the potential to clinically affect cyclosporine metabolism which may result in increased cyclosporine blood levels and an increase in its toxic side effects, and likewise may also affect tetrahydrocannabinol and its metabolite levels in man.