The nonpsychoactive plant cannabinoid, (-)-cannabidiol, modulates in vivo responses to Delta(9)-tetrahydrocannabinol. We have found that cannabidiol can also interact with cannabinoid CB(1) receptor agonists in the mouse vas deferens, a tissue in which prejunctional cannabinoid CB(1) receptors mediate inhibition of electrically evoked contractions by suppressing noradrenaline and/or ATP release. Cannabidiol (0.316-10 microM) attenuated the ability of (R)-(+)-[2,3-dihydro-5-methyl-3-(4-morpholinylmethyl)pyrrolo-[1,2,3-de]-1,4-benzoxazin-6-yl]-1-naphthalenylmethanone (R-(+)-WIN55212) to inhibit contractions in a concentration-related, surmountable manner with a K(B) value (120.3 nM) well below its reported cannabinoid receptor CB(1)/CB(2) K(i) values. Cannabidiol (10 microM) also antagonized (-)-cis-3-[2-hydroxy-4-(1,1-dimethylheptyl)phenyl]-trans-4-(3-hydroxypropyl)cyclohexanol (CP55940; K(B)=34 nM) and [D-Ala(2), NMePhe(4), Gly-ol]enkephalin (DAMGO; K(B)=5.6 microM) and attenuated contractile responses to noradrenaline, phenylephrine and methoxamine but not to beta, gamma-methyleneadenosine 5'-triphosphate. At 3.16-10 microM, it increased the amplitude of evoked contractions, probably by enhancing contractile neurotransmitter release. We conclude that cannabidiol antagonizes R-(+)-WIN55212 and CP55940 by acting at prejunctional sites that are unlikely to be cannabinoid CB(1) or CB(2) receptors.