SUMMARY

Progesterone levels in the blood of ewes were assayed chemically. No progesterone was detected in blood from the jugular vein of any ewe. It was found in blood from the vein draining the active ovary during the oestrous cycle. Detectable amounts appeared on the 3rd day, and the mean concentration increased to about 1\m=.\8\g=m\g/ml.on the 7th day. This level was maintained until the 16th day and fell to <0\m=.\15\g=m\g/ml. on the 17th or last day of the cycle. Blood from the vein draining the active ovary during pregnancy showed a mean level similar to that reached during the oestrous cycle until about the 17th week when it gradually fell to, and remained at, <0\m=.\15\g=m\g/ml.a few days before parturition. In blood from the vein draining the pregnant horn of the uterus, progesterone was detected in low concentrations between the 9th and 18th weeks of pregnancy in only six out of 143 cases. Considerable variation between ewes in progesterone concentration was found. The measurement of individual hormones at known physiological states of the organs they influence is probably an essential step towards an understanding of endocrine balances and their relationship to bodily functions