Concentrated "surfactant" was obtained by centrifugation of alveolar wash from adult rabbits for 1 hour at 4°C, 1,000xG, and deposited in the trachea of premature rabbit fetuses, killed at the gestational age of 28 days, i.e., 1 day before appreciable amounts of surface active phospholipids are naturally present in the pulmonary fluid. Pressure-volume tracings revealed a mature pattern in the surfactant-treated animals, with wide hysteresis. Significantly less air entered the lungs of control fetuses from the same litters, and the pressure-volume tracings from these animals showed a premature pattern, with minimal hysteresis. The lungs of the fetuses were fixed in formalin under the endotracheal deflation pressure of 10 cm H20. Histological examination of the lungs revealed moderate or prominent air expansion of the parenchyma in surfactant-treated animals, whereas no or only slight air expansion characterized the controls. Tracheal deposition of surfactant prior to the first breath thus seems to enhance the air expansion of the premature lung, and might possibly be applied as a method for prophylaxis against the idiopathic respiratory distress syndrome, which is related to deficiency of surface active phospholipids in the pulmonary fluid.