To achieve an isolated, ventilated, perfused newborn rabbit lung preparation, newborn rabbits are anesthetized and ventilated. The pulmonary vasculature is perfused with a Krebs-Ringer bicarbonate solution containing glucose and plasma expanders, and the lungs are isolated in a 37 degrees C chamber at 100% humidity. The success of a perfusion is evaluated on three levels. 1) Visual. 2)mechanical: a, drop in flow of greater than 20% indicates failure; b, an increase of more than 25% of the initial lung weight indicates fluid accumulation and failure. 3) Biochemical: a, the endogenous ATP concentration after perfusion; b, the level of the enzyme choline phosphotransferase (CPT) after perfusion. Isolated, perfused, ventilated newborn rabbit lungs maintained for 4 h with no changes in the monitored physical and mechanical parameters have an endogenous ATP level of 1.06 +/- 0.06 (2 SD) mumol ATP/mg wet weight and a CPT level of 1.34 +/-0.15 (2 SD nmol [14C]CDP-choline per milligram protein in 30 min. These values are stable at a level lower than observed in intact, hypoxic, newborn rabbit's lungs. In constrast, if perfusion is maintained after any of the monitored criteria indicate failure, the endogenous ATP concentration and CPT activity are significantly depressed.