The purpose of this study was to examine how lungs in early postnatal animals that are administered bromodeoxyuridine (BrdU), a thymidine analog known to be a cytogenetically toxic agent, change in adult life. BrdU was injected (1 mg/g body weight) intraperitoneally in suckling rats on day 6 after birth. Immunohistochemical studies revealed that BrdU was incorporated in alveolar cells, and infrequently in the mesothelium, endothelium, and bronchiolar epithelium. The number (per high-power field) of alveolar cells incorporating BrdU was 8.76 +/- 0.50 in the interstitium and 1.66 +/- 0.25 in the epithelium. The animals were sacrificed at 2 and 8 weeks and 12 months of age. Control animals were treated the same as the BrdU-treated animals except for the injection of BrdU. Body weight, body size (nose-tail length), and lung weight were identical between BrdU-treated animals and control animals at all experimental periods. The histopathology and physiology of these lungs were studied. At 2 weeks of age, the BrdU-treated animals appeared morphologically and functionally similar to control animals. At 8 weeks and 12 months of age, the lungs in the BrdU-treated animals showed abnormal enlargement of the airspace, few alveoli, and less complexity in pulmonary architecture with or without increased lung compliance. These findings suggest that lung injured cytogenetically in early postnatal life may develop functional and structural abnormalities in later life.