SUMMARY: The progesterone content of a sample of plasma from the vein draining the ovary containing a corpus luteum and a sample of jugular venous plasma from each of two pregnant ewes, as estimated by the chemical method of Edgar & Ronaldson [1958], was compared with the 'progestin' content of the same sample, estimated by the biological method of Hooker & Forbes [1947]. It appeared that all, or almost all, the progestational activity of the ovarian venous plasma could be accounted for in terms of progesterone, but that the jugular venous plasma, in which no progesterone was detected chemically, may have contained progestins other than progesterone. A discussion of the Hooker-Forbes technique for estimating progestins is included.