During puberty the endometrium undergoes cyclic changes morphologically defined as a phase of growth, secretion, and exfoliation. They occur as a result of periodically recurring changes in the activity of the hypothalamic-pituitary-ovary, which results in fluctuations in the concentration of estrogen and progesterone in the blood. The sexual cycle, where there has been no fertilization of the ovum, the corpus luteum hormone action expires, decreased serum estradiol and progesterone in the blood, in consequence of which comes to shedding of the endometrium and menstrual bleeding. Discusses the theories explaining the mechanism of menstrual bleeding in the light of experimental studies.