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Heifers were used to evaluate progesterone metabolism in the liver and brain during the estrous cycle. Blood samples were collected from the carotid artery and jugular, portal and hepatic veins. Progesterone concentrations in the hepatic vein were less than in portal and jugular veins and the carotid artery, but there were no differences between the carotid artery and jugular and portal veins. Progesterone concentrations in the hepatic, portal and jugular veins represented 25, 89 and 86 percent of concentrations in the carotid artery, respectively. During the estrous cycle much of the progesterone in blood is metabolized by the liver.