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SUMMARY The effect of milk reabsorption upon lactation was studied by giving daily intraperitoneal injections of milk to lactating mice during the first 5 days of lactation. The weight gain of the young of mice injected with mouse or cow milk was similar but was less than the gain of the young of the non-injected control mice. The weight gain of the young of the mothers receiving lipid-free cow milk was similar to that of the controls, whereas that of the young of mothers receiving lipid-free cow milk with added lipids was again less than in the control group. There was no difference in litter mortality rate between the controls and the group treated with lipid-free cow milk, nor between the groups treated with mouse, cow and lipid-free cow milk with added lipids; the mortality, however, in the three latter groups was higher than in the two former ones. The injection of milk decreased milk yield. The effect of the injection of milk was observed within 24 h of the first injection. The results suggest that milk secretion is impaired by the injection of milk. The effect may be ascribed to the lipid fraction of milk or to some unknown chemical factor within it. The rapid onset of the effect suggests that it is not mediated by an immunological reaction.

The effect of adrenaline on the maternal and foetal plasma ACTH concentration of twelve pregnant sheep with chronically implanted vascular catheters has been studied. Adrenaline infused into the jugular vein of the ewe or foetus produced carotid arterial adrenaline concentrations of 1-9 ng/ml. The foetal plasma ACTH was 253 +/- 73 pg/ml and it showed a fivefold increase during adrenaline infusion; the ACTH concentration achieved was proportional to the plasma adrenaline. In the ewe plasma ACTH was 99 +/- 23 pg/ml. During adrenaline infusion to the ewe this rose by an amount dependent on the adrenaline concentration achieved and there was also a rise in foetal plasma ACTH but no consistent change in foetal plasma adrenaline. There was no reproducible change in plasma corticosteroid concentration during adrenaline infusion into the foetus but a rise in maternal plasma corticosteroid concentration during infusion into the ewes. Because the adrenaline concentrations achieved during the infusions were within the physiological range, the results indicate that circulating catecholamines may directly or indirectly influence the concentration of ACTH in the circulation. Also, physiological rises in plasma catecholamines in pregnant animals may stimulate the release of ACTH from the foetal pituitary.

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