Peripheral Plasma Progesterone Levels During the Ovine Estrous Cycle

Abstract

Plasma progesterone levels were determined daily for approximately 26 days in 7 cyclic ewes. Circulating progesterone concentration was lowest on day 2 (0.1 ng/ml) with a small increase through day 5 (0.4 ng/ml). Progesterone levels rose from 0.6 ng/ml on day 6 to 1.1 ng on day 9; an abrupt rise was observed on day 10 (2.4 ng/ml) and was maintained through day 16. Progesterone concentration.declined sharply on day 17 to 0.6 ng/ml; this was followed by the occurrence of estrus within 24 hr. Considerable day-to-day variation of progesterone concentration was noted, particularly during the period of peak luteal function. (Endocrinology85: 11, 1969)