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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