CONCENTRATION OF PROGESTIN IN THE SERUM OF THE RABBIT DURING PREGNANCY, THE PUERPERIUM AND FOLLOWING CASTRATION12

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Abstract
Progestin levels were determined in the serum of rabbits by the Hooker-Forbes technique. A rapid increase was seen during the first 12 days of pregnancy and slower rise from the 12th day till term. A maximum concentration of 10 ug/ml of serum was noted just prior to parturition. The serum progestin decreased after delivery of the young, indicating that a fall in the absolute progestin level is not essential for parturition. Castration led to a drop in serum progestin and abortion in all instances. It was noted that the rate of loss of progestin from the blood following (a) parturition, (b) castration of pregnant rabbits and (c) castration and hysterectomy of pregnant rabbits was markedly slower than that observed after the intravenous injection of progesterone.