An investigation was made into the influence of a hemodynamic factor on pulmonary expansion in the syndrome of neonatal secondary pulmonary atelectasis (with or without hyaline membranes). Heart-lung specimens from human neonates with such pathological conditions were used. BaSC>4 solution was injected intratracheally, intravascularly, or by both routes combined. The results obtained by these various techniques are discussed with reference to radiological, microradiological and histological features. The experiments, indicate not only a disturbance in ventilation in the syndrome of secondary pulmonary atelectasis (with or without hyaline membranes) but also a disturbance in pulmonary hemodynamics.