Although chronic prenatal hypoxia is considered a major cause of persistent pulmonary hypertension of the newborn, experimental studies have failed to consistently find pulmonary hypertensive changes after chronic intrauterine hypoxia. We hypothesized that chronic prenatal hypoxia induces changes in the pulmonary vasculature of the chicken embryo. We analyzed pulmonary arterial reactivity and structure and heart morphology of chicken embryos maintained from days 6-19 of the 21-d incubation period under normoxic (21% O₂) or hypoxic (15% O₂) conditions. Hypoxia increased mortality (0.46 vs.0.14; P<0.01) and reduced the body mass of the surviving 19-d embryos (22.4±0.5 g vs. 26.6±0.7 g; P<0.01). A decrease in the response of the pulmonary artery to KCl was observed in the 19-d hypoxic embryos. The contractile responses to endothelin-1, the thromboxane A₂ mimetic U46619, noradrenaline, and electrical-field stimulation were also reduced in a proportion similar to that observed for KCl-induced contractions. In contrast, no hypoxia-induced decrease of response to vasoconstrictors was observed in externally-pipped 21-d embryos (incubated under normoxia for the last 2 days). Relaxations induced by acetylcholine, sodium nitroprusside or forskolin were unaffected by chronic hypoxia in the pulmonary artery but femoral artery segments of 19-d hypoxic embryos were significantly less sensitive to acetylcholine than arteries of control embryos (pD₂: 6.51±0.1 vs. 7.05±0.1, P<0.01). Pulmonary vessel density, percent wall area, and periarterial sympathetic nerve density were not different between control and hypoxic embryos. In contrast, hypoxic hearts showed an increase in right and left ventricular wall area and thickness. We conclude that, in the chicken embryo, chronic moderate hypoxia during incubation transiently reduced pulmonary arterial contractile reactivity, impaired endothelium-dependent relaxation of femoral but not pulmonary arteries and induced biventricular cardiac hypertrophy.