PURPOSE: We investigated the temporal alterations of apoptosis and mitotic death followingirradiation in the rat's small intestinal crypts. MATERIALS AND METHODS: Male Sprague-Dawley rats were irradiated 2 Gy by 6 MV linear accelerator and sacrified at 2, 4, 8, 24, 48 hours after irradiation. The mean numbers of the apoptotic cells and mitotic cells per their small intestinal crypts were measured in the unirradiated control and irradiated groups. To compare with H & E staining, ISEL (In Situ End Labelling) were performed in the group having the highest apoptotic count. RESULTS: The mean number of the apoptosis per crypt in the control group was 0.14 and those at 2, 4, 8, 24, 48 hours after irradiation were 1.43, 3.19, 1.15, 0.26, 0.17, respectively. So the apoptosis development was increased upto 4 hours and then normalized around 24 hours following irradiation. The mean number of the mitotic cells per crypt in the control group was 1.29 and those at 2, 4, 8, 24, 48 hours after irradiation were 0.56, 0.47, 0.23, 0.65, 1.19, respectively. The mitotic cell counts following irradiation was decreased to 8 hours and recovered to the normal level about 48 hours. So the increment of apoptotic cell count was occurred earlier and more remarkable than the decrement of mitotic cell count after irradiation. According to the staining time, false positivity was found in the ISEL staining. CONCLUSIONS: The cell death in the small intestinal crypt developed by acute radiation damage was usually decreased to the normal level within 24~48 hours after irradiation and the apoptosis was thought to be more important process than the mitotic death.