Most of what we know concerning the physiology of menstruation pertains to its endocrinology, for the fundamental rôle in the mechanism of this process, like that of most other vegetative functions, is still played by the endocrine system rather than by the more highly developed nervous apparatus which is so important in such volitional processes as skeletal muscle contractility.The endocrine factors are probably important even in the fetal and prepubertal epochs, but at puberty their function becomes much more important and more conspicuous. Much has already been learned concerning the respective rôles played by the two ovarian secretions, folliculin and progestin, in the reproductive cycle, and much is being learned as to the relation, apparently subordinate, of these two hormones to the two corresponding sex hormones of the anterior hypophysis.Such menstrual abnormalities as excessive functional bleeding can not be studied in mere terms of endometrial histology or even of quantitative blood hormone content. A broader viewpoint, based on considerations of comparative physiology, helps much in the interpretation of such problems. For example, periodic hemorrhage from the genital canal may be of various types, each with an entirely different mechanism, as discussed in the paper. The most common variety, encountered clinically as functional bleeding with hyperplasia of the endometrium, is apparently due to a persistence and excess of the follicle secretion, with deficiency or excess of the corpus luteum hormone, progestin. There can be little doubt, however, that this immediate ovarian disturbance is in turn due to an imbalance of the governing sex hormones of the pituitary.Recent work on the problem of uterine motility indicates that this is subject to a definite cyclical influence, that folliculin is in general a stimulator and progestin an inhibitor of the rhythmic uterine contractions. I have therefore advanced the view that the pain of primary dysmenorrhea is explainable, in part at least, on the basis of these facts. This pain is characteristically of a colicky type, suggesting spasmodic muscle contraction, and it begins characteristically a day or two before the onset of menstrual bleeding, i. e., just at the time that the withdrawal of the inhibiting corpus luteum hormone takes place. While various other factors undoubtedly play a rôle in the production of primary dysmenorrhea (psychogenic, constitutional, developmental, etc.) the immediate cause would seem to be in a heightened irritability of the uterine muscle. On this basis, a very tentative suggestion as to a plan of organotherapy is made, to be combined, of course, with measures directed toward other such factors as those mentioned above.