A method was developed for recording isometric tension from isolated small arteries (mean internal diameter, 169 μm) and veins (mean internal diameter, 273 μm) of the term fetal lung and was then applied to the study of the mechanisms controlling perinatal pulmonary hemodynamics. The specific purpose was to determine whether the activity of the prostaglandin synthetic system in vessels is conditioned by the oxygen tension and the mode of action of endothelin-1. Both preparations appeared structurally intact and, after normalizing their lumen diameter to either the transmural pressure in vivo (artery) or the contractile capacity of the vessel in vitro (vein), generated force to the activating solution (5 mM Ca2+in K+Krebs' solution) in excess of the expected performance under physiological conditions. Treatment with indomethacin (2.8 μM) had no effect on arteries preequilibrated at low Po2(21±1 mm Hg); however, the same treatment contracted (≈45% of the response to activating solution) arteries at either an intermediate (40±0.8 mm Hg) or high (70±0.9 mm Hg) Po2. Endothelin-1 contracted both arteries and veins in a concentration-dependent manner, the threshold being lower with veins (1–10 versus 10–100 pM). Endothelin-1 constriction was also seen in arteries whose tone had been raised with a thromboxane A2analogue, whereas in thromboxane-treated veins constriction was preceded by a modest relaxation over the range of 1–1,000 pM. The findings with indomethacin lead us to infer that small pulmonary arteries are endowed with a prostaglandin-relaxing mechanism that becomes functional on raising the PO2from fetal to neonatal levels. Endothelin-1 is a constrictor regardless of the level of intrinsic tone, suggesting a possible role of the peptide in maintaining elevated pulmonary vascular tone in the fetus.