The pattern of nuclear degeneration of oocytes and the effect of follicle stimulating hormone (FSH) on the profile of nuclear degeneration of oocytes in the follicles at the stage of Type 4, 5, 6 or 7 was studied. Mouse ovaries were examined by serial histological sections. Nuclear degeneration of oocytes was found among follicles at Type 4-7 stage of development and the pattern of nuclear degeneration was strongly correlated with the developmental stage of follicule or oocyte. Nuclear degeneration of oocytes was observed in 52.2% of Type 4-5 (preantral) follicles and 39.2% of Type 6-7 (antral) follicles. In the Type 4-5 follicles, degeneration, such as pycnotic changes and disappearance of chromosomes, was dominant. In Type 6-7 follicles, however, about 80% of oocytes induced pseudo-maturation division during the degenerating processes. Pseudomaturation was observed in the oocytes more than 51 μm in diameter. The administration of FSH significantly increased the number of Type 6-7 follicles, but not Type 4-5 follicles, and decreased the number of both Type 4-5 and Type 6-7 follicles with degenerating oocytes. A large number of macrophages were identified immunohistochemically in the interstitial tissue around the follicles. Macrophages make a cluster around the atretic follicles, but were not identified inside of the follicles containing degenerating oocytes. These results suggest that the pattern of oocyte degeneration depends on the developmental stage of follicles and oocytes, and that FSH accelerates the process of degeneration of oocytes, and that macrophages are involved in the process of removal of degenerating oocytes.