Objective    To investigate ultrastructural pathology and pathogenesis of radiation-induced pulmonary injury.    Methods    Sixty-three adult male SD rats were divided in three groups randomly

receiving irradiation at dose of 10

20

and 30 Gray at their right hemithorax. The irradiated rats of each group were killed at 1

3

7

14

28

90

180 d

and their lungs were observed by light and electron microscope. Another 14 rats served as normal control.     Results    In early phase of irradiation-induced injury (1-30 d)

we observed type Ⅰ pneumocytes injury

increased and activated macrophages

the activation of type Ⅱ pneumocytes

the damage and proliferation of endothelial cells

the congregation of platelet and RBC

thrombosis

and interstitial edema. In the fibrotic phase (60-180 d)

there were the abnormal proliferation of type Ⅱ pneumocytes

the proliferation of fibroblasts and collagens

the atrophy and depletion of capillary vessels

and finally lung fibrosis.    Conclusion    The pathological changes in endothelial cells of capillary vessels

the disturbance of blood circulation and interstitial edema are the early major pathological changes of irradiation-induced pulmonary injury

which may be the primal factors responsible for the development of irradiation-induced lung fibrosis. The process of lung fibrosis involves  fibroblast activation

abnormal proliferation of type Ⅱ pneumocytes and increased macrophages

resulting from the combined effects of complex factors.