Levels of Progesterone in Peripheral Blood Plasma During the Estrous Cycle of the Bovine

Progesterone was measured in 108 samples of peripheral blood plasma from 12 animals by a double-isotope derivative method, to define changes during the estrous cycle. Progesterone averaged 16.2 0.8 ng/ml (10 -~) of blood plasma. Significant (P < .01) differences were obsecved between averages for two-day intervals during the estrous cycle. Plasma progesterone averaged 13.3 -----2.0, 10.1 1..1, 9.9 1.1, and 10.6 +---1.7 ng/ml, respectively, for 1 to 2 days before estrus, the day of estrus, and two and six days after estrus. Similarly the averages were 18.4 3.3, 20.4 .3.1, 25.8 3.9, and 19.4 1.8 ng/ml for ]o. 12, and 14 days after estrus and 3 to .1 days before the subsequent estrus. Averages for 4 and 8 days after estms and 5 to 6 before the subsequent estrus ranged frmn 17.1 to 17.7 ng/ml. Averages for progesterone concentration were significantly higher for Day 14 (P < .05) and three to four (P < .01) and five to six (P < .05) days before a subsequent estrus, as con,pared to the day of estrus and 2 and 6 days after estrus. Similarly, the Day 12 average was significantly (P < .05) higher than averages for the day of estrus and two days after estras, and Days 3 to 4 before estrus averaged significantly (P < .05) higher than 1 to 2 days before estrus.