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SUMMARY The involution of the mammary glands of lactating rats, which normally follows the cessation of suckling, was greatly retarded over a period of 9 days by administering oxytocin to the mothers, following removal of the litters on the 4th day of lactation. This effect was obtained with a commercial extract of the natural hormone, the same extract without preservative (benzethonium chloride), and with synthetic oxytocin. Vasopressin administered under the same conditions had a less well-marked effect. No retardation of mammary involution could be obtained with oxytocin in the absence of the anterior pituitary gland. Similar results were obtained by administering prolactin to the mothers, but growth hormone (GH) had only a slight effect in maintaining the mammary glands. When both prolactin and GH were given, the maintenance of gland structure was particularly marked. A majority of the animals receiving synthetic oxytocin showed vaginal mucification which is taken to be indicative of the presence of a luteotrophic hormone (prolactin). These results are discussed in relation to the possible role of oxytocin in the release of prolactin and other lactogenic and galactopoietic hormones from the anterior lobe.

The mammogenic potency of miroestrol, an oestrogenic substance isolated from the tuberous roots of the leguminous plant Pueraria mirifica, was estimated as 0\m=.\70 (with 5% fiducial limits of 0\m=.\40and 1\m=.\18) relative to oestradiol in the ovariectomized rat, and as 2\m=.\2 (with 5% fiducial limits of 1\m=.\2and 4\m=.\9 relative to oestrone in the mouse. The effects of miroestrol on the body weight and weights of the endocrine glands, with the exception of the thyroid in the rat, were qualitatively similar to those of oestradiol and oestrone. In the rat oestradiol (0\m=.\1\g=m\g/day) inhibited the increase both in body weight and thyroid weight that occurred after ovariectomy, whereas miroestrol (0\m=.\1\g=m\g/day) inhibited only the increase in body weight.

SUMMARY Adult female rats undergoing their first lactation were experimentally weaned by removing their pups on day 4 of lactation. The initial phase of mammary gland involution began with the removal of protein granules from the milk by digestion in large stasis vacuoles in the alveolar epithelial cells. Later stages involved a progressive necrosis of the epithelial cells by auto-phagocytosis. Removal of fat droplets and cellular debris occurred at this time and was accomplished by macrophage-like cells. Finally, epithelial cells were detached from the basement membrane and were found, in varying stages of degeneration, lying free in the lumina of the alveoli.