Plasma progesterone levels throughout the ovarian cycle of the common marmoset (Callithrix jacchus)

Radioimmunoassay of progesterone in marmoset plasma has been used to determine ovarian cycle length. Total cycle length was 30.1 +/- 3.8 days (mean +/- SD, n = 30, range 24-41 days, median 29.5 days). The pre-ovulatory (follicular) phase, during which progesterone levels were below 10 ng/ml, lasted for 8.8 +/- 3.7 days (mean +/- SD, n = 30, range 3-20 days, median 8.5 days). The post-ovulatory (luteal) phase, during which progesterone levels were greater than 10 ng/ml, lasted for 21.5 +/- 2.2 days (mean +/- SD, n = 30, range 14-29 days, median 21.5 days). Total cycle length was almost twice that recorded in an earlier study. The reasons for this difference are discussed.