A P2Y receptor with 65% identity to mammalian P2Y6 receptors, termed the p2y3 receptor, was recently cloned from a chick brain cDNA library and was proposed to represent a novel P2Y receptor subtype [Mol Pharmacol 50:258-265 (1996)]. We cloned the turkey homologue of the chick p2y3 receptor, which shares high sequence identity (97.6%) with the chick receptor, and we stably expressed this receptor and the rat P2Y6 receptor in 1321N1 human astrocytoma cells. The capacities of uridine and adenine nucleotides to promote inositol phosphate accumulation and intracellular Ca2+ mobilization were determined for both receptors. UDP and 5-bromo-UDP were the most potent agonists and UTP was a less potent full agonist at both receptors. In contrast, adenine nucleotides and nucleotide derivatives were relatively more potent at the turkey p2y3 receptor than at the rat P2Y6 receptor. To determine whether the avian p2y3 receptor defined a new subtype of mammalian P2Y receptor or was a species homologue of the mammalian P2Y6 receptor, we screened two different human genomic libraries and a Southern blot with a p2y3 receptor probe, under low-stringency conditions that allowed the clear identification of the human P2Y6 receptor gene. Our data indicated that the human genome does not contain a receptor that is more homologous to the avian p2y3 receptor than the P2Y6 receptor. Taken together, these data further define the pharmacological selectivities of these UDP-selective receptors and strongly suggest that the avian p2y3 receptor is a species homologue of the mammalian P2Y6 receptor.