Extracellular nucleotides have been shown to induce vasodilatation of conductance arteries by release of the endothelium-derived hyperpolarizirg factor (EDHF), As small resistance arteries are of greater importance for blood pressure regulation, a whole rat mesenteric arterial bed preparation was used in the present study when evaluating the physiological relevance for EDHF in mediating nucleotide dilatation. Dilatory responses were examined after pre-contraction with noradrenaline in the presence of 10 mm indomethacin. Adenosine-5'-O-thiodiphosphate (ADPbetaS), adenosine triphosphate (ATP) and uridine triphosphate (UTP) induced vasodilatation (pEC(50) = 6.5-7 and E-max = 40-70%), while uridine diphosphate (UDP) was ineffective. Endothelium-derived hyperpolarizing factor was studied in the presence of 0.5 mm Nomega-nitro-L-arginine (L-NOARG). ADPbetaS and UTP induced strong and potent EDHF-dilatations, while ATP only had a weak effect (E-max = 25%). After P2X(1) receptor desensitization with 10 muM alpha,beta-methylene-adenosine triphosphate, the ATP response was significantly increased (E-max = 65%). Putatively, this could be because of simultaneous activation of both endothelial P2Y receptors and P2X(1) receptors on smooth muscle cells, which resulted in the release of EDHF and subsequent hyperpolarization, and depolarization, respectively. Nitric oxide (NO) was studied in the presence of 60 mm K+. ADPbetaS, ATP and UTP induced weak NO dilatations, suggesting a minor role for NO as compared with EDHF, In conclusion, extracellular nucleotides stimulate dilatation by activating P2Y(1) and P2Y(2)/P2Y(4) receptors, but not P2Y(6) receptors. The dilatory responses are mediated primarily by EDHF in the peripheral vascular bed.