Evidence suggests that Delta9-tetrahydrocannabinol (THC) may have antihypertensive effects and that the vasodilator effect of endocannabinoids is enhanced in rats made hypertensive by chronic NO synthase inhibition. Therefore, the aims of the present study were to investigate whether the in vitro and in vivo cardiovascular responses to THC are altered by Nomega-nitro-L-arginine methyl ester (L-NAME) treatment. The vasorelaxant effects of THC were enhanced in small mesenteric arteries from L-NAME-treated rats. This enhanced response was not inhibited by cannabinoid CB1 receptor antagonism [1 microM N-(piperidin-1-yl)-5-(4-iodophenyl)-1-(2,4-dichlorophenyl)-4-methyl-1H-pyrazole-3-carboxamide; AM251]. Pretreating vessels with the transient receptor potential vanilloid receptor receptor agonist capsaicin at 10 microM for 1 h reduced vasorelaxation to THC to a greater extent in L-NAME-treated than control rats. Inhibition of cyclooxygenase with 10 microM indomethacin inhibited THC responses in arteries from L-NAME-treated rats but not from control rats. In conscious, chronically instrumented rats, 1 mg kg-1 i.v. THC caused a pressor effect, with vasoconstriction of the renal and mesenteric vascular beds, and hindquarters vasodilatation. Pretreatment with 3 mg kg-1 i.v. AM251 reduced the pressor and vasoconstrictor effects of THC, abolished the hindquarters vasodilatation, and revealed a bradycardic response. L-NAME-treated rats showed similar pressor and vasoconstrictor responses to THC, but with bradycardia, and reduced hindquarter vasodilatation. These data show that, in vitro, isolated arteries of L-NAME-treated rats show enhanced vasorelaxant responses to THC through an increased sensory nerve component and stimulation of prostanoids. However, in vivo, THC causes a CB1 receptor-mediated pressor effect with hindquarters vasodilatation. There was no evidence of enhanced vasodilator effects of THC in L-NAME-treated animals in vivo.