ABSTRACT Background:The lungs represent the major part of respiratory system. It consists mainly of spongy tissue and conducting air tubules called bronchi and bronchioles. The spongy tissue at which gaseous exchange takes place between air and blood are forming alveolar duct and alveoli The aim of this study: is to evaluate the changes that occur in the structure of the lung in the postnatal period and correlate it with clinical applications. Material and methods: 12 pregnant female rats of Wistar strain were used in this study. The study was carried out on pups of female rats after mating with male rats. Pups were given water and balanced diet by gastric intubation. Pups of pregnant rats were sacrificed at 14th and 21th days postnatal. Lung from all the age groups were processed for light microscope examination. Results:At day 14H&E stained sections revealed that the rat lung was formed of air saccules divided by large numbers of secondary septa. The wall of bronchioles consists of ciliated cuboidal epithelium. All primary and secondary septa showed the double capillary network. Masson’s trichrome stainedsections showed ordinary amount of collagen fibers in the wall of the bronchioles and bronchial vessels.At day 21H & E stained sections of lung revealed normal architecture of alveoli with many alveolar sacs.The double capillary layers inside the primary and secondary septa are transformed into a single central capillary layer. Masson’s trichrome stained sections showing average amount of collagen fibers in the wall of the bronchi. Conclusion: There are changes occur in the structure of lung in the postnatal period at different age groups.