Progesterone levels in the Japanese monkey (Macaca fuscata fuscata) during the breeding and nonbreeding season and pregnancy.

Progesterone levels were determined in the peripheral blood of the Japanese monkey during the menstrual cycle, breeding season, nonbreeding season, and gestation. Progesterone levels averaged less than 0.3 ng/ml in the earlier follicular phase, rose to maximum values of 3.8 ng/ml approximately eight days before menses, and declined to less than 1.0 ng/ml before menstruation. Progesterone levels during the nonbreeding season were comparable with those during the early follicular phase. In pregnant animals circulating progesterone either continued to rise around day 20, or fell as in the infertile cycle, then rose again. Progesterone levels declined between days 35 and 60 and were variable until about day 130 of gestation.