Oleamide (cis-9-octadecenoamide) exhibits some cannabimimetic responses despite its low affinities at the currently known cannabinoid receptors. Here we have investigated whether or not it is a vasorelaxant in rat small mesenteric arteries. Oleamide elicited vasorelaxation (EC =1.2±0.2 μM, R =99.1±3.9%, n=8) which was reduced by endothelial removal. Nitric oxide synthase inhibition reduced the response (E 5.3±1.6 μM, R =59.2±7.7%, n=7; P<0.01) as did blockade of Ca -sensitive K channels (K ) with apamin plus charybdotoxin (both 50 nM) (EC =2.1±0.2 μM, R =58.4±1.9%, n=5; P<0.05). Desensitisation of vanilloid receptors with capsaicin (10 μM for 30 min) shifted the oleamide concentration-response curve ∼30-fold to the right (n=7; P<0.01). Pertussis toxin (400 ng ml for 2 h) caused a two-fold shift in the response curve (EC =2.2±0.4 μM, R =66.8±4.5%, n=6; P<0.01). Rimonabant (CB cannabinoid receptor antagonist; SR141716A; 3 μM) significantly inhibited relaxation induced by oleamide (EC 3.5±0.3 μM, R =75.1±1.9%; n=8; P<0.05). In contrast, neither the more selective CB receptor antagonist, AM251 (1 μM), nor the CB antagonist, SR144528 (1 μM), had significant effects. O-1918 (10 μM), a putative antagonist at a novel endothelial cannabinoid receptor (abnormal-cannabidiol site), markedly reduced the relaxation to oleamide (n=7; P<0.01). It is concluded that oleamide responses in the rat isolated small mesenteric artery are partly dependent on the presence of the endothelium, activation of Ca -sensitive K channels (K ) and involve capsaicin-sensitive sensory nerves. Oleamide may share a receptor (sensitive to rimonabant and O-1918, and coupled to K(Ca) and G( ) with anandamide in this vessel. This might be distinct from both of the known cannabinoid receptors and the novel abnormal-cannabidiol site. © 2006 Nature Publishing Group All rights reserved.