This study was performed to characterize the responsiveness of rat juxtamedullary afferent arterioles to receptor-selective P2-purinoceptor agonists. Experiments were performed in vitro using the blood perfused juxtamedullary nephron technique which permits measurement of afferent arteriolar inside diameter by videomicroscopy. Rats were pretreated with enalaprilat (2 mg; i.v.) for 30 minutes prior to kidney isolation. Renal perfusion pressure was set at 1 lOmmHg and held constant. Afferent arteriolar diameter averaged 22.6±0.8/im (n=34) under control conditions. Stimulation with 0.1. 1, 10 and 100/iM ATP (n=10) elicited a concentration-dependent vasoconstriction averaging 7.8±2.2, 17.2±2.3, 21.3+3.8 and 23.1 ±5.3%; respectively. Administration of the P-selective agonist, α, β-methylene ATP(n=9), significantly reduced afferent diameter from 6.5±1.9, 7.7±2.2, 9.5 ±3.3 and 5.8±2.1%; respectively, over the same concentration range. Similar treatment with the Pjy-selective agonist, 2-methylthio ATP, reduced afferent diameter by 7.3±2.5, 9.1±2.2, 4.7±1.2 and 4.2±1.2%; respectively. During administration of UTP, which is a potent agonist of P2u-purinoceptors, afferent arteriolar diameter increased slightly hy 6.6±2.3% (P>0.05 vs con) to O.VM UTP and decreased by 5.0±4.1, 31.1±8.3 and 71.7±8.1% to 1, 10 and 100μM UTP; respectively. The rank order potency of the vasoconstriction induced by 100μM agonist is UTP>ATP≥aJ3-methylene ATP=2-methylthio ATP. The following data suggest the presence of multiple P2 receptors on juxtameduliary afferent arterioles and are consistent with classification of those receptors as being of the P2x, P2y and P2u receptor subtypes.