The maturation division of oocytes has been studied in relation to estrus, and follicle diameter in gonadotropin-treated and untreated beef cattle. Injection of pregnant mare serum (PMS) gonadotropin had no effect on the pattern of maturation division of oocytes. The follicles reached full size shortly before the onset of estrus. During early estrus, all oocytes had nuclei of metaphase I; the maturation division rapidly progressed to metaphase II during late stages of estrus. Human chorionic gonadotropin (HCG) treatment which has been recognized to be effective for stimulating ovulation did not affect the pattern of maturation division of the oocytes. Degeneration of oocytes occurred in all stages of follicular maturation during and following estrus, irrespective of the follicle size.