A classification system for oocytes and follicles in the mouse ovary is proposed to provide a standardized terminology for describing ovarian components. This classification is based on three main criteria: (1) the size of the oocyte, (2) the number of follicle cells in the follicular envelope, and (3) the morphology of the follicle. Oocytes are categorized into small, growing, and large based on their diameter. Follicles are divided into small, medium, and large, with further subdivisions based on the number of follicle cells and their morphology. Small follicles include types 1, 2, and 3a, where type 3a has a complete ring of follicle cells surrounding a small or growing oocyte. Medium-sized follicles include types 3b and 4, with type 3b having a complete ring of follicle cells around a growing oocyte and type 4 having two layers of follicle cells. Large follicles include types 5a, 5b, 6, 7, and 8, with type 8 representing the preovulatory follicle. This classification has been used in quantitative analysis of the ovary for total oocyte counts and to determine the normal distribution of follicle stages at different ages or after treatment. The classification provides a more accurate description of follicle development stages and is useful for evaluating and comparing changes in oocyte and follicle growth under various conditions. The proposed system aims to standardize terminology and facilitate the comparison of results from studies on ovarian development and follicle growth.A classification system for oocytes and follicles in the mouse ovary is proposed to provide a standardized terminology for describing ovarian components. This classification is based on three main criteria: (1) the size of the oocyte, (2) the number of follicle cells in the follicular envelope, and (3) the morphology of the follicle. Oocytes are categorized into small, growing, and large based on their diameter. Follicles are divided into small, medium, and large, with further subdivisions based on the number of follicle cells and their morphology. Small follicles include types 1, 2, and 3a, where type 3a has a complete ring of follicle cells surrounding a small or growing oocyte. Medium-sized follicles include types 3b and 4, with type 3b having a complete ring of follicle cells around a growing oocyte and type 4 having two layers of follicle cells. Large follicles include types 5a, 5b, 6, 7, and 8, with type 8 representing the preovulatory follicle. This classification has been used in quantitative analysis of the ovary for total oocyte counts and to determine the normal distribution of follicle stages at different ages or after treatment. The classification provides a more accurate description of follicle development stages and is useful for evaluating and comparing changes in oocyte and follicle growth under various conditions. The proposed system aims to standardize terminology and facilitate the comparison of results from studies on ovarian development and follicle growth.