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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.

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