We have studied, by a nonisotopic in situ DNA end-labeling (ISEL) technique, the frequency of apoptosis in the external granular layer (EGL) of the cerebellum after whole-body irradiation of newborn mice by gamma-rays from 60Co. The total number of normal cells and cells showing morphological features of apoptosis were counted. The frequency of apoptotic cells was expressed as a percentage of the total number of cells in EGL. The extent of changes following 2 Gy (10.9 Gy/min) was studied at 2, 4, 6, 8, 12, or 24 hours after exposure. The maximal frequency was found 6-8 hours after exposure. Newborn mice that received 0.18, 0.36, 0.54, 1.08, 1.98, or 3.96 Gy were examined 6 hours after irradiation. Measurements performed after irradiation showed a dose-related increase in apoptotic cells in each of the mice studied. The dose-response curves were analyzed by a linear- quadratic model; frequency was (13.49 +/- 1.175) D + (-1.52 +/- 0.334) D2 +/- 0.048 (r2 = 0.981, D = dose in Gy). It provides the basis required for a better understanding of results which will be obtained in any further studies for biological responses of radiation using newborn orfetal mice.