The effects of PGF2α and PGE1 on pulmonary vascular resistance in the intact sheep were investigated using a new right heart catheterization technique. PGF2α increased lobar arterial pressure but did not change pressures in the small lobar vein or left atrium. PGE1 decreased lobar arterial pressure but did not alter pressure in the small lobar vein or the left atrium. These results indicate under conditions of controlled blood flow PGF2α increases pulmonary vascular resistance by constricting vessels upstream from the small lobar veins whereas PGE1 decreases pulmonary vascular resistance by dilating upstream vessels, presumably the lobar arteries. Neither prostaglandin altered systemic arterial pressure. It is concluded that in the sheep lung the precapillary vessels are responsive to prostaglandins E1 and F2α.