[en] This research consists of two parts. In one part irradiation was used to determine the radiosensitivity of prenatal female germ cells. Mice were given 18 rads of ⁶⁰Co γ radiation (1 rad/min) at various times in utero. The effect of this treatment was measured by light microscopic enumeration of unilaminar follicles remaining in the ovary at 49 days after birth. Greatest sensitivity was seen for irradiation on day 11 post conception. However the germ cells at this period were not as sensitive as are dictyate oocytes of the juvenile mouse. The other part of this study involved γ irradiation of 14-day-old female mice. Electron microscopy was used to examine the ultrastructural changes taking place in the dictyate oocyte as a response to the radiation. Among the observed changes were irregular nuclear shape, nuclear membrane swelling, chromatin clumping, increased numbers of lysosomes, and enlarged lysosomal areas. The significance and degree of these changes are discussed