Total normothermic venoarterial bypass was established in 6 healthy pigs over a period of 18 hours. A heparin-coated closed extracorporeal system was used and no heparin was administered systemically. During the bypass period the main pulmonary artery was occluded and the heart was maintained in a beating state. All the animals maintained stable hemodynamics and normal blood gases during the entire period of bypass. In the postbypass period, the central hemodynamics continued to be stable while the arterial oxygen tension (inspired oxygen fraction = 0.21) decreased significantly (p less than or equal to 0.05). The total body oxygen uptake, on the other hand, remained unaltered. All the animals died within 4 hours after weaning off the venoarterial bypass circuit on account of pulmonary edema in 2 and cardiac arrest in 4. Death was preceded by progressive pulmonary hypertension and lactacidosis in all the animals. Histological examination of the lungs showed pulmonary parenchymal damage ranging from interstitial edema to intraalveolar hemorrhage and parenchymal necrosis involving more than 80% of the pulmonary parenchyma. A normothermic total venoarterial bypass of 18 hours duration or more produces pulmonary edema of varying severity, pulmonary hypertension, pulmonary parenchymal necrosis, and lactacidosis in healthy juvenile pigs, resulting uniformly in their death. Despite these sequelae the systemic arterial hypoxemia may only be mild to moderate.