The appearance of mammalian follicles at the time of ovulation has been described by numerous observers. Importance features to which attention has been directed are: changes in the vascularization of the follicle, the appearance of an avascular area known as the macula pellucida or stigma, the formation of small blood clots near the stigma and some extravasation of blood, and the rupture of the follicle at the stigma and exist of the oocyte and follicular fluid. The first major advance beyond this descriptive stage came with the demonstration that antral pressure does not increase prior to follicle rupture. This observation eliminated a number of hypotheses concerning ovulation and focused attention on the deterioration and weakening of the follicle wall at the stigma. That the follicle wall weakness prior to rupture is established beyond doubt by gross observations of its increased fragility during various manipulative procedures, by stress-strain measurements in vivo and in vitro, and by histological observations. Progressive weakening of the stigma region by thinning and degeneration is a fundamental aspect of the preovulatory maturation of follicles, and it would appear to be a necessary prerequisite to rupture.