The effect of thoracic irradiation on antibiotic penetration into the alveolar space was determined in a hemithoracic irradiation rat model to evaluate radiation-induced acute alveolar injury at various time intervals. The results of this investigation may be summarized as follows: (1) The transfer of ceftazidime (CAZ: Modacin) from blood to lung tissue, that is, the permeability of pulmonary capillary epithelium, and the transfer of ceftazidime from lung tissue to bronchoalveolar lavage fluid, that is, the permeability of the alveolar epithelium, peaked at 4 to 5 weeks after thoracic irradiation; (2) the time course of change in the absolute concentration of ceftazidime in lung tissue showed a significant increase not only in the irradiated lung but also in the contralateral non-irradiated lung 3 days or more after irradiation. The finding that the administration of antibiotics may cause a significant increase in drug concentration in lung tissue even in the contralateral lung at 3 days after irradiation suggests that the change induced at the alveolar level immediately after irradiation affects the non-irradiated lung field through an as yet unknown mechanism.