1. Responses of the carotid artery of rabbits to intraluminal and abluminal administration of purinergic agonists were examined. The carotid artery was perfused in vitro and changes in diameter were recorded. 2. Intraluminal acetylcholine, ADP, and ATP produced pronounced vasodilatation, whereas abluminal acetylcholine, but not ADP and ATP, produced dilatation of phenylephrine-preconstricted arteries. Intra- and abluminal adenosine and nitroprusside produced equipotent vasodilatation. 3. N omega-nitro-L-arginine abolished dilator responses to acetylcholine and adenine nucleotides, and unmasked vasoconstrictor responses to high concentrations of these agonists. Responses to adenosine and nitroprusside were not affected by nitro-L-arginine. 4. Intraluminal, but not abluminal, administration of nucleotidase-resistant adenine nucleotide analogues 2-methylthio-ATP and ADP beta S produced significant vasodilation in arteries preconstricted with phenylephrine. Intra- and abluminal administration of alpha,beta-methylene-ATP, a potent P2X-purinoceptor agonist, did not produce vasodilatation in preconstricted arteries. 5. Abluminal ADP failed to elicit dilatation of phenylephrine-preconstricted arteries even in the presence of the ADPase inhibitor beta,gamma-methylene-ATP (10(-5)M). When P2X-purinoceptors, which mediate adenine nucleotide-induced vasoconstriction, were stimulated with alpha,beta-methylene-ATP (10(-5)M), abluminal ADP produced vasodilatation, presumably because P2X-purinoceptors were occupied, thereby unmasking P2Y-purinoceptor-mediated dilatation. 6. These results suggest that asymmetric vascular responses of rabbit carotid arteries to adenine nucleotides may be due in part to preferential activation of P2Y-purinergic receptors on endothelium and P2X-purinergic receptors on vascular smooth muscle.