The changes occurring in the cellular environment of differentiating type II pneumocytes have been studied using the perinatal rat lung as a model. The various cellular types in contact with type II pneumocytes during late fetal and neonatal period have been identified. The percentage of type II cells in contact with each cellular type has been calculated, and the extent of the contact areas has been measured. During the studied period, type II pneumocytes are in contact with other epithelial cells (type I or type II pneumocytes) on their lateral sides. On their basal side, they are facing lipid loaded fibroblasts and endothelial cells of the capillary walls. The results of the measurements show that from day 19 of gestation on, the contact area between type II pneumocytes and other epithelial cells decreases, whereas the luminal and especially the basal sides increase. On day 19, the whole basal side of type II pneumocytes in lined by lipofibroblasts. As maturation proceeds thereafter, contact is gradually made with endothelial cells. But the extent of these contacts remains always smaller than the contacts between type Ii pneumocytes and lipofibroblasts. The developmental significance of these results is discussed.