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SUMMARY Oxytocin has been assayed in the jugular vein blood of goats during parturition; for comparison a few measurements were also made during pregnancy. The hormone was extracted from blood plasma by gel filtration, followed by lyophilization and then assayed in the lactating guinea-pig by the increase in intramammary pressure after intra-arterial injection. No oxytocin could be detected in the blood during pregnancy and it was found in only one of eight goats studied during the first stage of labour. The hormone was present in appreciable quantities in blood taken during the second stage of labour, and in general, the concentration rose to a maximum when the head presented. In cases of twin births oxytocin was usually present in the blood during the birth of the second kid but at a concentration lower than during delivery of the first. After expulsion of the kid the blood oxytocin titre diminished rapidly, suggesting that secretion of oxytocin ceased as soon as the kid was born. In three experiments the total release of oxytocin during a considerable portion (2·7–11·0 min.) of the second stage labour was estimated as 223–726 m-u. The results are consistent with the view that oxytocin is not essential for the induction of labour. Rather the hormone is released in response to stimuli arising from distension of the vagina and vulva, and by virtue of its contractile effect on the uterus assists parturition. The half-life of intravenously injected oxytocin in the lactating goat was found to be 1 min. 22 sec. After storage of lyophilized blood extracts at −15° for 5 months milk-ejection activity had declined by only 27%.

The ability of human pituitary growth hormone (HGH) to induce milk formation in the pseudopregnant rabbit and to stimulate the crop gland of the pigeon has been investigated. Assayed against purified sheep prolactin, fourteen preparations of HGH all showed marked lactogenic activity in the rabbit mammary intraductal test and the majority of preparations were highly active. On the other hand, in the pigeon, the classical test aminal for prolactin assay, low activity in stimulating the crop gland was usual and only two preparations showed activity comparable with that shown by purified sheep prolactin. The finding that HGH has consistently high activity in a mammalian lactogenic test, which may in suitable circumstances provide good estimates of potency, is discussed in the light of the possibility that HGH may be the only human pituitary lactogen.

Summary 1. The arterio‐venous technique, hitherto favoured for mammary gland studies, is subject to drawbacks which make it unsuitable for quantitative work, as many conflicting results attest. This technique in its present form is capable of yielding only the broadest qualitative conclusions. It is claimed that one source of uncertainty is minimized by the use of anaesthetized animals. Studies on the isolated perfused udder and surviving tissue slices have given useful results; tracer isotopes, so far hardly used in this field, may solve many outstanding problems. 2. Mammary slices respire fairly actively compared with other tissues. In the rat there is a marked increase in respiration at parturition, correlated with hormonally controlled changes in mammary gland metabolism, and the respiration further increases with increasing milk yield. Glucose and mannose are the only sugars so far found which increase the oxygen uptake of rat mammary gland slices above endogenous values. 3. Studies in vivo and in vitro alike indicate a high respiratory quotient for the lactating gland which may mean the synthesis of fat from oxygen‐rich precursors. Studies in vitro suggest that in the non‐ruminant the fat precursor may be carbohydrate while the ruminant gland probably synthesizes fat from acetate. Non‐lactating gland gives an R.Q. less than unity and the hormonally controlled events associated with parturition are accompanied by a rapid increase in the R.Q. to values greater than unity. 4. The source of energy of the active gland is not yet clear; there is evidence that carbohydrate, long‐chain fatty acids, β‐hydroxybutyric acid and acetate can be oxidized. 5. Mammary gland slices show appreciable anaerobic glycolysis (acid production) which is decreased in presence of oxygen. It is possible that part of the aerobic acid production may be due to formation of citric acid or fatty acids. 6. Enzymes recently detected in mammary tissue comprise desoxyribonuclease, catalase, xanthine dehydrogenase, and acid phosphatase. Most attention has been given to the alkaline phosphatase and arginase, both of which occur abundantly in lactating gland in certain species. The concentration of alkaline phosphatase in the rat mammary gland increases rapidly during late pregnancy to a constant high level which is maintained throughout lactation, indicating that the enzyme is concerned with mammary function rather than growth. Histoenzymatic studies have suggested that this enzyme is mainly localized in the myoepithelium and capillary endothelium, there playing a role associated with transport of metabolites, rather than in the secretory epithelium, but this needs further confirmation. Changes in the arginase levels of the mamma in the rat during lactation suggest that deamination processes probably associated with gluconeogenesis from protein or amino‐acids play an important role in mammary metabolism in this species. The arginase content of the mamma is low in herbivores (including ruminants) so that deamination in the mammary gland must be le...