Progesterone levels were studied in the peripheral blood of the bonnet monkey (Macaca radiata) during the menstrual cycle and gestation. Progesterone levels averaged less than 0.5 ng/ml during the first 9 days of the cycle. A significant rise began on about day 10 of the cycle or 16 days before the onset of menses. Maximal values were obtained by day 18 of the cycle or 9 days before menstruation. Progesterone levels declined to less than 1.0 ng/ml plasma 2 to 3 days before menstruation. Although progesterone levels were higher in the pregnant animals during the first 12 days postbreeding as compared to the luteal phase of the nonpregnant animal, the general profile was similar. The point of divergence of progesterone levels in pregnant vs nonpregnant animals occurred on day IS of gestation or day 24 of the menstrual cycle, at which time levels in pregnant animals were maintained and levels in nonpregnant animals declined significantly. The abrupt divergence of progesterone levels in nonpregnant animals suggests that there is a welldefined period during the cycle in which regression of the corpus luteum begins if the animal is not pregnant. A decline in progesterone concentration in pregnant animals which began at about day 35 continued until day 45 or 50 postbreeding. Progesterone levels were quite variable from day 50 until about day 145 to 150 of gestation (2–8 ng/ml) at which time levels began to increase. During the interval from day 150 to parturition, progesterone levels increased 3-, 5-, 10-, and 19-fold in the 4 animals studied, with the greatest increase noted the last 6 days prior to delivery. Progesterone levels, an average of 20 ng/ml plasma about 12 hr before delivery, declined to 2 ng/ml by 12 hr postpartum.
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