Data from several studies are presented which suggest that blood flow to the corpus luteum may be important in the regulation of this gland. Blood flow to the luteal ovary increases from less than 1 ml/mm to 3 to 7 ml/min as the corpus luteum develops and is maintained. During regression, blood flow to the luteal ovary declines sharply. Data obtained with radioactive microspheres indicates that the changes in blood flow to the luteal ovary can be attributed to changes in flow to the corpus luteum, which receives the majority of the blood. In addition, the entrapment of 15 µ microspheres was less than that for 50 µ microspheres during the late luteal phase of the cycle suggesting that arteriole-venule shunting of blood occurs. As a result of the decline in total ovarian blood flow and the shunting of blood within the corpus luteum, there appears to be a severe restriction in the quantity of blood available to the luteal cell during regression. Morphological data obtained during this period indicate that there is even a decrease in the relative volume of the capillary network within the corpus luteum.