The effect of pretreatment with cannabidiol (CBD; 50 mg/kg i.p.) on the distribution of radioactivity in chromatograms of ethyl acetate extracts of the brains of mice injected with tritiated Delta(1)-tetrahydrocannabinol ((3)H-Delta(1)-THC; 1.0 mg/kg i.v.) was determined. The pretreatment with CBD produced significant increases in the levels of radioactivity in the brain assigned to Delta(1)-THC and its centrally active metabolite 7-hydroxy-Delta(1)-THC: the changes produced were respectively 1.4 and 2.0 fold.Pretreatment with CBD did not bring about any detectable change in the degree of 'catalepsy' produced by Delta(1)-THC. This negative finding may have been due to the wide limits of error that were obtained in the bioassay.