Female B6C3F1 mice were irradiated on day 17 prenatal age, or day 0, 7, 35, 105, 240 or 365 postnatal age with 0.95, 1.9, 2.85, 3.8 or 5.7 Gy of gamma-rays from 137Cs. They were allowed to live out their entire life spans under specific pathogen free conditions. All the mice were given autopsies at death and were examined histologically for neoplastic and non-neoplastic diseases. The mice in the early postnatal period were most sensitive to the life-shortening effect of radiation. The shortening effect of irradiation given during the late fetal period was almost the same as that given during the young adult period. Incidences of lung, liver, pituitary, ovarian and bone tumors and malignant lymphoma of the lymphocytic type increased after irradiation of mice in the late fetal period. Mice in the early postnatal period are more susceptible to the induction of liver and ovarian tumors and malignant lymphoma of the lymphocytic type than are fetal mice. Myeloid leukemia and Harderian gland tumor did not develop in excess when mice were irradiated in fetal or in neonatal period; whereas, these neoplasms were induced by irradiation during the adult period.