1 The P2 receptors that mediate contraction of the rat isolated small (SPA, 200 ± 500 mm i.d.) and large (LPA, 1 ± 1.5 mM i.d.) intrapulmonary arteries were characterized. 2 In endothelium-denuded vessels the contractile order of potency was a,b-methyleneATP (a,bmeATP)44UDP=UTP=ATP=2-methylthioATP4ADP in the SPA and a,b-meATP=UT-P5UDP42-methylthioATP, ATP44ADP in the LPA. a,b-meATP, 2-methylthioATP and ATP had signi®cantly greater e ects in the SPA than the LPA (P50.001), but there was no di erence in the potency of UTP or UDP between the vessels. 3 In the SPA, P2X 1 receptor desensitisation by a,b-meATP (100 mM) inhibited contractions to a,bmeATP (10 nM ± 300 mM), but not those to UTP or UDP (100 nM ± 300 mM). In the LPA, prolonged exposure to a,b-meATP (100 mM) did not desensitize P2X receptors. 4 Pyridoxalphosphate-6-azophenyl-2',4'-disulphonic acid (PPADS), suramin and reactive blue 2 (RB2) (30 ± 300 mM) inhibited contractions evoked by a,b-meATP. UTP and UDP were potentiated by PPADS, una ected by RB2 and inhibited, but not abolished by suramin. 1 and 3 mM suramin produced no further inhibition, indicating suramin-resistant components in the responses to UTP and UDP. 5 Thus, both P2X and P2Y receptors mediate contraction of rat large and small intrapulmonary arteries. P2Y agonist potency and sensitivity to antagonists were similar in small and large vessels, but P2X agonists were more potent in small arteries. This indicates di erential expression of P2X, but not P2Y receptors along the pulmonary arterial tree.