Female rate were irradiated with 190R of X-rays at 10 days of age and ovarian ultrastructures were studied 4 and 7 months after irradiation. Ultrastructural changes were found in germinal epithelial cells, fibroblasts in the tunica albuginea and interstitial cells. The germinal epithelial cells exhibited various signs of degeneration but no sign of proliferation. Electron density of their basal part was reduced considerably. Their mitochondria became swollen and free ribosomes decreased in number. The nuclei often protruded from the free surface of these cells. These cells frequently fragmented and,finally, the complete desquamation occurred. The basement membrane became unevently thickened. Nuclei of enlarged fibroblasts in the tunica albuginea became irregular ellipsoid in shape, and the nuclear envelope was occasionally invaginated. Various cytoplasmic organelles of the fibroblasts were well-developed. Some abnormal invasion of cytoplasm into the nucleus was found in the interstitial cells showing the ultrastructural characteristics of steroid hormone synthesis. Various cytoplasmic organelles and inclusions invaded into the nucleus of these cells and the nuclear envelope sometimes disappeared locally. These interstitial cells contained a large number of irregular-shaped electron dense mitochondria with vesicular cristae, and numerous dilated vesicles of smooth-surfacedendoplasmic reticulum (SER). The cells of anovular follicles in the irradiated ovariesresembled, in fine structure, the granulosa cells in normal primary follicles of the non-irradiated ovaries. These cells seem to be less affected by early postnatal irradiation.