1 Vasorelaxation and hyperpolarization of endothelial cells by adenosine 5 0 -[b-thio]diphosphate (ADPbS) and adenosine 5 0 -[g-thio]triphosphate (ATPgS) were studied in rat-isolated mesenteric artery. Effects from stimulation of P2X receptors were avoided by desensitization with a,b-methylene adenosine triphosphate. 2 ADPbS caused concentration-and endothelium-dependent relaxations of methoxamineprecontracted small (third generation) and main mesenteric artery. These were inhibited by N o -nitro-L-arginine methyl ester (L-NAME) or a combination of apamin plus charybdotoxin (inhibitors of Ca 2+ -activated K + channels); L-NAME, apamin and charybdotoxin applied together abolished the response. 3 ATPgS induced limited relaxation (35% of methoxamine-induced tone at 10 mM) of small mesenteric artery, which was sensitive to L-NAME or endothelium denudation. However, it almost completely relaxed the main mesenteric artery over an extended concentration range (46 orders of magnitude) in an endothelium-dependent manner. This relaxation was inhibited by either L-NAME or a combination of apamin with charybdotoxin, and abolished by a combination of all the three inhibitors. 4 The P2Y 1 receptor antagonist MRS 2179 (2 0 -deoxy-N 6 -methyladenosine 3 0 ,5 0 -bisphosphate; 0.3 -3 mM) caused parallel rightward shifts of the concentration/relaxation curve to ADPbS (pA 2 ¼ 7.1). However, MRS 2179 did not inhibit, but potentiated, relaxant responses to ATPgS. MRS 2179 did not affect the contractile responses ATPgS in small mesenteric artery; ATPgS did not contract the main mesenteric artery. 5 ADPbS hyperpolarized the endothelium of the main mesenteric artery in a concentrationdependent manner. This was unaffected by L-NAME but antagonized by MRS 2179. ATPgS also hyperpolarized the mesenteric artery endothelium in a concentration-dependent manner but, when ATPgS was applied at 10 mM, its effect was potentiated by MRS 2179 (3 mM). 6 It is concluded that both relaxation and hyperpolarization to ADPbS are mediated by P2Y 1 receptors and that the endothelial hyperpolarization is related to the L-NAME-resistant relaxation. Relaxation to the P2Y 2 agonist ATPgS shows regional variation along the mesenteric vasculature. The mechanisms for potentiation of relaxation and hyperpolarization by ATPgS are unknown, but may indicate interactions between P2Y receptor subtypes.