The genitalia of a control series of nineteen animals, slaughtered for other reasons than reproductive failure, were studied. Of these six were in various stages of pregnancy, one was in a ‘proliferative phase’, being slaughtered probably just before the first oestrus after calving, and twelve represented different phases of the oestrous cycle, more than half exemplifying the last 4 days before heat. Stages were judged from the appearance of the ovaries, and checked in five instances by repeated rectal examinations, and observation of behaviour during life. Of the nineteen uteri eight yielded bacteria on culture, sometimes in moderately high density; from two of them, pure cultures were recovered respectively of Pseudomonas and Neisseria catarrhalis; and in another, probably Proteus was found. No previous records of these three genera at such a site have been found. Only aerobic blood agar plate cultures, and those for tuberculosis organisms were made. Dissection results unequivocally supported the view of Tagliavini in opposition to that taken by Hammond, that the sanguineous elements in post-oestral discharge originated from endometrial extravasation. The cow slaughtered 4 days after heat indicated that congestion disappears from the caruncles before leaving the areas between them. Microscopically, no mast cells, as observed by the Italian workers, could be seen; it appeared that a ‘proliferative phase’ occurs in every cycle during the three pre-oestral days, when gland tissue proliferates from its nadir of development, surface epithelium grows in height, and vascularization progresses. The rate and interrelations of these changes seemed variable. Arterioles appeared to be withdrawn from the superficial mucosa during the secretory phase. Tagliavini's claim to have observed sloughing of epithelium about the 17th day, strictly equivalent to the process of menstruation in the Primates, must on the evidence be regarded with considerable reserve.