The authors propose a classification system for oocytes and follicles in the mouse ovary to standardize terminology and facilitate the comparison of developmental stages under normal and experimental conditions. The classification is based on three main criteria: the size of the oocyte, the number of cells in the follicular envelope, and the morphology of the follicle. Oocytes are categorized into small (diameter < 20 μ), growing (diameter 20-70 μ), and large (diameter 70 μ). Follicles are divided into small, medium, and large groups, further subdivided based on the number of follicle cells and their morphological appearance. This classification has been used for quantitative analysis of oocyte counts and the distribution of follicle stages at different ages or after treatment.The authors propose a classification system for oocytes and follicles in the mouse ovary to standardize terminology and facilitate the comparison of developmental stages under normal and experimental conditions. The classification is based on three main criteria: the size of the oocyte, the number of cells in the follicular envelope, and the morphology of the follicle. Oocytes are categorized into small (diameter < 20 μ), growing (diameter 20-70 μ), and large (diameter 70 μ). Follicles are divided into small, medium, and large groups, further subdivided based on the number of follicle cells and their morphological appearance. This classification has been used for quantitative analysis of oocyte counts and the distribution of follicle stages at different ages or after treatment.