In human pulmonary vascular preparations, precontracted arteries were more sensitive to the relaxant effect of acetylcholine (ACh) than veins (pD2 values: 7.25±0.08 (n=23) and 5.92±0.09 (n=25), respectively). Therefore, the role of prostacyclin (PGI2) was explored to examine whether this mediator may be responsible for the difference in relaxation. In the presence of the cyclooxygenase (COX) inhibitor, indomethacin (INDO), the ACh relaxations were reduced in arteries but not in veins. On the contrary, an inhibitor (L‐NOARG) of the nitric oxide synthase blocked preferentially the relaxation in veins. A greater release of 6‐keto‐PGF1α, the stable metabolite of PGI2, was observed in arterial preparations than in venous preparations when stimulated with either ACh or arachidonic acid (AA). Exogenous PGI2 produced a reduced relaxant effect in the precontracted vein when compared with the artery. In the presence of the EP1‐receptor antagonist AH6809, the PGI2 relaxation of veins was similar to arteries. In veins, AA (0.1 mM) produced a biphasic response, namely, a contraction peak (0.4–0.5 g) followed by a relaxation. These contractions in venous preparations were abolished either in the absence of endothelium or in the presence of INDO or an EP1‐receptor antagonist (AH6809, SC19220). In the arterial preparations AA induced only relaxations. In both vascular preparations, COX‐1 but not the COX‐2 protein was detected in microsomal preparations derived from homogenized tissues or freshly isolated endothelial cells. The differential vasorelaxations induced by ACh may be explained, in part, by a more pronounced production and release of PGI2 in human pulmonary arteries than in the veins. In addition, while PGI2 induced relaxation by activation of IP‐receptors in both types of vessels, a PGI2 constrictor effect was responsible for masking the relaxation in the veins by activation of the EP1‐receptor. British Journal of Pharmacology (2004) 142, 788–796. doi:10.1038/sj.bjp.0705843