PURPOSE: To evaluate imaging findings and lung density changes after 95% oxygen inhalation in rat. MATERIALS AND METHODS: A total of 18 rats were divided into three groups on the basis of inhalation time: group I(n=6) inhaled 95 % oxygen for 24 hours, and group II(n=6) for 48 hours, group III(n=6) for 60 hours. A control group(n=6) inhaled room air(21% oxygen). Chest radiograph and high resolution computed tomography were performed, and pathologic and imaging findings were compared. RESULTS: Chest radiograph showed abnormality only in group III. High resolution CT, however, revealed abnormal findings in all three groups : diffuse ground glass opacity in groups I, II and III, additional focal patchy consolidation at the peripheral portion in group II, and diffuse consolidation in group III. Lung density was sig-nificantly higher in group I than in controls(p <0.05), while density in group II was not significantly different from that in group I(p >0.05). In group III, density was significantly higher than in group II. The lung density changes seen in all groups showed a bilateral diffuse increased pattern. but, in group III, changes were more severe in the central, peripheral and posterior portion of the lower lung. Ground glass opacity and focal patchy consolidaton seen on HRCT were found on pathologic examination to be due to alveolar cell hyperplasia and septal thickening. Consolidation was caused by alveolar edema and hemorrage. Pathologic lesions were randomly distributed in both lungs. CONCLUSION: One HRCT images, rat exposed to hyperoxia showed ground glass opacity, patchy consolidation and diffuse consolidation. Depending on exposure time, the pathologic findings also indicated increased lung density and a bilateral, diffuse distribution pattern, as well as alveolar cell hyperplasia and septal thickening, alveolar edema and hemorrage. HRCT may be more helpful than simple X-rays for the early detection of pulmonary oxygen toxicity.