Pregnancies were prolonged by daily subcutaneous injection of progesterone to the maternal rats from day 20 through 24 of gestation (Term is 22 days). The fetuses were harvested by cesarean section on gestation days (GD) 21 through 25. In spite of reduction in placental weight the fetal body weight increased during prolonged gestation. Both lung wet and dry weights decreased resulting in smaller lung per body weight. Compared to term fetuses, post term fetuses (GD 25) exhibited a 10% reduction in lung cell numbers, and a 90% reduction in lung glycogen. Lung disaturated phosphatidyl choline expressed per lung increased up to GD 23 and subsequently decreased by GD 25. Surface tension of lung extracts and pulmonary lavage fluids increased with prolongation of gestation. Histological studies revealed a decrease in the number of lamellar bodies in the Type II cells and a gradual reduction in the size of terminal sacs leading to areas of complete atelectasis at GD 25. It is suggested that lung hypocellularity, depletion of lung glycogen, reduction in intracellular reserve of phospholipids, loss of pulmonary fluid leading to atelectasis, and loss of surface activity of pulmonary fluid may be predisposing factors for the development of respiratory distress in postmaturity.