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A method was adapted for the quantitative determination of progesterone obtained from the peripheral plasma of several domestic animals (cow, gilt and ewe). The technique utilized the detection of a chloroacetate derivative of 20β-hydroxypregn-4-en-3-one (enzymically converted from progesterone) by gas—liquid chromatography with electron capture detection. An improved extraction procedure is presented in which the plasma extract is saponified instead of the plasma; this was particularly effective for the removal of the yellow pigments normally found in large amounts in bovine plasma and allowed one-step isolation of progesterone by thin-layer chromatography. Progesterone levels in the peripheral plasma during the follicular phase of the oestrous cycle of the cow, gilt and ewe averaged 0·4 ng./ml., 0·5 ng./ml. and 0·2 ng./ml., respectively. Luteal phase levels of progesterone averaged 6·6 ng./ml., 27·1 ng./ml. and 2·1 ng./ml. in the cow, gilt and ewe, respectively. Progesterone increased during gestation in one cow from 4·7 (sixth month) to 6·1 ng./ml. plasma (eighth month). No evidence was obtained for a pre-ovulatory rise of progesterone in one cow during the 12 hr. immediately following oestrus.

It is suggested that corpus luteum function can be monitored in domestic animals during the oestrous cycle by the daily estimation of progesterone in the peripheral plasma.