The concentrations of estradiol, estrone, testosterone, and progesterone in the systemic plasma of 9 female rhesus monkeys during the intermenstrual period were measured by radioimmuno and competitive protein binding assays. A synchronous midcycle surge of estrogen and testosterone was followed by a sharp decline as progesterone rose. During the luteal phase of the cycle, testosterone dropped significantly (707 ± 89 pg/ml, mean ± SE) from the values of the follicular phase (836 ± 99 pg/ml, p < 0.01). 15 prevented the midcycle elevation of estradiol and testosterone whereas cholesterol administered in the same way and used as a control had no effect. In addition, progesterone reduced the basal levels of estradiol (39 ± 6.3 pg/ml, mean ± SE) from those in the luteal phase (71 ± 14.5 pg/ml, p < 0.05) and in the pretreatment follicular phase 71 ± 3.6 pg/ml, p < 0.01). When the amounts of estradiol in ovarian venous plasma were compared with those in the systemic circulation, the latter were highly correlated with the estradiol from the ovary with the largest follicle (r = 0.91), as well as with that from the ovary on the contralateral side (r = 0.85). Testosterone levels in ovarian venous plasma although higher than those in plasma drawn systemically, were not correlated with systemic concentrations. The data suggest that progesterone occupies a key regulatory position in the primate by altering the biosynthetic pathways involved in estrogen secretion around the time of ovulation.
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