Objective: We investigated the morphologic changes within 24 hours after a single ${gamma}$-irradiation in the rat brain. Methods: Forty Sprague-Dawley rats were used. After a burr hole trephination on right parietal area, cerebral hemisphere was irradiated with 2Gy and 5Gy using iridium-192($^{192}Ir$), respectively. The effect was assessed at 4, 8, 12 and 24 hours after irradiation. The histological changes were scored following the detection of edema or disarray severity. TUNEL-positive cells exhibiting apoptotic morphology were counted in irradiated region. Results: Cortical edema and disarray were initially showed at 4 or 8 hour and almost all defined at 24 hour after irradiation. And the injury was wedge shape. TUNEL-positive cells were minimal at 8 hour after irradiation as the number of positive cells were $2.6{pm}5.27$(n=5) after 2Gy, and $0.8{pm}0.84$(n=5) after 5Gy. But, the number of apoptotic cells were increased markedly to $60{pm}6.24$ at 12 hour after 2Gy and to $104{pm}19.7$ at 24 hour after 5Gy. Conclusion: There were prominent morphologic changes immediately after ${gamma}$-irradiation. And, apoptosis was increased according to the time period. These findings implicate that brain irradiation induces rapid apoptotic change, which may play an important role in the pathogenesis of radiation-induced pathologic conditions.