The hormonal induction of pseudo-menstrual periods on several occasions in a human $ castrate helps to clarify certain aspects on the mechanism of uterine bleeding. Failure to obtain sufficient endo-metrium for study within 1 hr. after onset of flow on 3 occasions following massive doses of estrogen-progesterone therapy furnished evidence of the minimal role played by the endometrium in bleeding. This case study further emphasizes the observations of Bartelmez that the myo-metrium and its vascular supply are dependent on endocrine control and are responsible for uterine bleeding. Withdrawal of estrogen-progesterone therapy was followed on several occasions by a menstrual flow but bleeding did not occur subsequent to withdrawal of large doses of progesterone (50-80 mg.) alone. These findings substantiated the contention that menstruation is the result of a sudden deprivation of estrogen or of progesterone in the presenec of estrogen and that pathologic bleeding is based on the same mechanism of fluctuating levels of ovarian hormones.