Cyclic Changes in Plasma Progestins and Estrogens in Squirrel Monkeys1

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Abstract
Serum progestin and 17β-estradiol concentrations were measured during the breeding season of S. sciureus. Serum concentrations of both estradiol and progestins were markedly higher than those reported for other primate species. In the cycles examined the peak values of estradiol were 503.4± 57.5 pg/ml and 399.5 ± 27.7 ng/ml for progestins. Each steroid fluctuated in a cyclic pattern. Based upon these cyclic fluctuations the reproductive cycle of the squirrel monkey was estimated to be approximately 9 days in length.