Levels of Progesterone in Peripheral Blood Plasma during the Estrous Cycle of the Ewe

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Abstract
Fifty peripheral venous blood samples were collected from 37 ewes in different stages of the estrous cycle to estimate changes in progesterone concentration. After separation of the plasma, progesterone levels were estimated by a double-isotope derivative method involving conversion of progesterone to 20 β-hydroxy-pregnene-3-one with sodium boro-hydride-3H. Progesterone levels increased from 22.2°2A ng./ml. (1O−9 gm./ml.) around estrus to 37.4°4.6 ng./ml. by 9 to 11 days (P<.01), and then declined to 21.8°3.0 ng./ml. 13 to IS days after estrus (P<.01).The average for the experiment was 27.7°5.2 ng./ml. with an average coefficient of variability of 41.7 %.