Virodhamine is a recently identified novel endocannabinoid. Cannabinoids may evoke vasorelaxation through novel receptors in the vasculature and/or through release of vasodilator peptides from sensory nerve endings. Virodhamine induced endothelium-dependent relaxation in the rat isolated small mesenteric artery mounted in a myograph and precontracted with methoxamine. Desensitization of vanilloid receptors by capsaicin did not affect relaxation responses to virodhamine. The CB(1) receptor antagonist SR 141716A (3 microM), but not the more CB(1)-selective blocker AM 251 (1 microM), attenuated the response, while two CB(2) receptor antagonists, SR 144528 (1 microM) and AM 630 (10 microM), had no effect. The novel antagonist for the putative endothelial 'abnormal-cannabidiol receptor', O-1918 (30 microM), inhibited virodhamine relaxations. Hence virodhamine may activate this novel receptor, which might also recognize SR 141716A. Inhibition of nitric oxide synthase (L-NAME 300 microM) did not affect relaxation to virodhamine but the responses were markedly reduced when tone was induced with 60 mM KCl, suggesting a role for the activation of K(+) channels. The Ca(2+)-activated K(+) channel (K(Ca)) blockers, apamin (50 nM) and charybdotoxin (50 nM), inhibited virodhamine vasorelaxation. Combination of these blockers with SR 141716A (3 microM) caused no further inhibition. It was concluded that virodhamine relaxes the rat small mesenteric artery by endothelium-dependent activation of K(Ca), perhaps via the putative abnormal-cannabidiol receptor.