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The effect of the plant-derived nonpsychotropic cannabinoid, cannabidiol (CBD), on the function of hydroxytryptamine (5-HT)_3A receptors expressed in Xenopus laevis oocytes was investigated using two-electrode voltage-clamp techniques. CBD reversibly inhibited 5-HT (1 μM)-evoked currents in a concentration-dependent manner (IC₅₀ = 0.6 μM). CBD (1 μM) did not alter specific binding of the 5-HT_3A antagonist [³H]3-(5-methyl-1H-imidazol-4-yl)-1-(1-methylindol-3-yl)propan-1-one (GR65630), in oocytes expressing 5-HT_3A receptors. In the presence of 1 μM CBD, the maximal 5-HT-induced currents were also inhibited. The EC₅₀ values were 1.2 and 1.4 μM, in the absence and presence of CBD, indicating that CBD acts as a noncompetitive antagonist of 5-HT₃ receptors. Neither intracellular BAPTA injection nor pertussis toxin pretreatment (5 μg/ml) altered the CBD-evoked inhibition of 5-HT-induced currents …