We compared the effects of acetylcholine (Ach) in intrapulmonary arteries and veins of adult sheep. Preconstricted arterial rings with endothelium relaxed with Ach whereas arteries without endothelium or pretreated with NG-nitro-L-arginine methyl ester (L-NAME), an inhibitor of nitric oxide (NO) synthesis, did not dilate. Venous rings with or without endothelium, whether under resting tension or preconstricted, always contracted with Ach. However, preconstricted veins dilated with bradykinin, another endothelium-dependent vasodilator. Preconstricted veins pretreated with indomethacin or SQ29548, a prostaglandin H2 (PGH2)/TxA2 receptor blocker, did not constrict but rather dilated with Ach. This dilation was abolished with removal of endothelium or treatment with L-NAME, indicating that endothelium-derived NO (EDNO) was mediating the dilation. We conclude that Ach is an endothelium-dependent vasodilator in ovine intrapulmonary arteries, whereas in veins, Ach elicits two responses: EDNO-mediated vasodilation and vasoconstriction mediated by TxA2/PGH2.