Ovarian and luteal blood flow rates were measured, by means of radioactive microspheres, in anesthetized rabbits on day 8 of pseudopregnancy before and after lowering the ovarian perfusion pressure with a sling placed around the aorta. When blood pressure was lowered by 42%, luteal flow decreased 9-fold while flow to the remaining part of the ovary remained unchanged, indicating the presence of an autoregulatory mechanism in the ovarian interstitial gland. Ovarian progesterone secretion, assessed from progesterone concentrations in ovarian venous blood, was positively correlated to the blood flow per unit of weight of luteal tissue. A high rate of luteal blood flow may be necessary for an optimal steroid production by the corpus luteum.