Plasma progesterone concentration at different stages of pregnancy was measured in the blood obtained from the uterine artery (6 goats), the uterine vein (6 goats), and the ovarian vein (11 goats). Progesterone concentration was also measured in the peripheral venous blood before and after extirpation of the ovaries and the uterus (16 goats). The concentration of progesterone was found to rise gradually in the peripheral and ovarian vein blood to reach a maximum at 90 days of pregnancy. This was followed by a gradual decline with consistent low values of about 7 ng prog./ml 3–4 days before parturition. A significant decrease in peripheral progesterone concentration was found 10 minutes after removing the ovaries and the uterus. In two goats sampled before and after parturition, the plasma progesterone concentration was found to decrease before parturition and to remain low for at least three days after parturition.