As an index of reproductive cyclicity, the levels of progestins and immunoreactive estrogens were determined in the serum of adult female marmosets. The estrogens were determined by radioimmunoassay and the progestins were measured by a protein binding assay. The estrogens increased from a mean value of 2.3± 0.2 ng/ml to a mean peak level of 6.2 ± 0.8 ng/ml in approximately 5 days, followed by a rapid decline. Progestins began to increase shortly after the initiation of the estrogen rise, attained a mean peak level of 272.3 ± 17.5 ng/ml in approximately 5 days, and declined to levels of 140.3 ± 6.8 ng/ml before the ensuing rise in serum estrogens. The mean cycle length, as determined by the interval between successive progestin peaks, was IS.5 ± 1.5 days. These results suggest the marmoset, a New World monkey, has a shorter reproductive cycle than Old World monkeys. (Endocrinology92: 1096, 1973)