The object of the experiment was to verify whether cannabidiol (CBD) reduces the anxiety provoked by d9-THC in normal volunteers, and whether this effect occurs by a general block of the action of d9-THC or by a specific anxiolytic effect. Appropriate measurements and scales were utilized and the eight volunteers received, the following treatments in a double-blind procedure : 0.5 mg/kg d9-THC, 1 mg/kg CBD, a mixture containing 0.5 mg/kg d9-THC and 1 mg/kg CBD, and placebo and diazepam (10 mg) as controls. Each volunteer received the treatments in a different sequence. It was verified that CBD blocks the anxiety provoked by d9-THC, however this effect also extended to marihuanalike effects and to other subjective alterations induced by d9- THC. This antagonism does not appear to be caused by a general block of d9-THC effects, since no change was detected in the pulse-rate measurements. Several further effects were observed typical of CBD and of an opposite nature to those of d9-THC. These results suggest that the effects of CBD, as opposed IO those of d9-THC. might be involved in the antagonism of effects between the two cannabinoids.