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LEE J. GROTA and KRISTEN B. EIK-NES

Summary.

Plasma progesterone concentrations in rats were determined by a method employing gas-liquid chromatography with electron capture detection. Progesterone concentration was found to decrease before parturition and to increase again during lactation. A maximal post-partum concentration of this hormone was found during the 4th day of lactation. Removal of the litter or the placenta at delivery resulted in a decrease in the progesterone concentration at this time. Injection of large doses of prolactin in hypophysectomized female rats was associated with an increase in plasma progesterone concentration.

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LEE J. GROTA, VICTOR H. DENENBERG and M. X. ZARROW

Summary.

Rat offspring were delivered by Caesarian section at term and fostered to lactating mothers which had had their own young removed. The major variables studied were the presence or absence of the placenta at the time of fostering, the rate of presentation of the foster young to the mother, and the length of time a mother was without any young. The percentage of animals surviving until weaning (21 days) and the body weight of the survivors were recorded.

The presence of the placenta at fostering resulted in a decrement in survival probability. The time that the foster mother was without pups and the rate at which foster pups were presented to the mother had an interactive effect: the highest survival rate was obtained when pups were presented 5 min apart to mothers which had had their own young removed 60 min before fostering.

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VICTOR H. DENENBERG, LEE J. GROTA and M. X. ZARROW

Summary.

The purpose of these experiments was to isolate some of the determinants of maternal behaviour in the rat by use of a fostering procedure. Four variables were investigated: the number of days the foster mother had been lactating prior to receiving foster young, the presence or absence of the placenta when young were fostered, the effects of administering oestradiol upon the behaviour of the foster mother, and the number of hours the young were with their own mother before being fostered. Body weights were obtained at 10, 15 and 21 days of age and the percentage alive at 21 days was recorded.

As the number of days the foster mother had been lactating increased, the mortality rate of the foster young also increased and the body weight of the survivors decreased. The absence of the placenta resulted in an increase in the mortality rate; a decrease in body weight occurred for young fostered without the placenta to 10-day-lactating mothers. The injection of oestradiol resulted in significant increases in survival percentages and body weights suggesting that the oestrogen content of the placenta might be significant in the maternal behaviour complex. All young fostered 12 hr after birth survived through weaning, thus suggesting that the mother's behaviour during the first few hours of life are critical both for the survival of the young and for the weight of the survivors.