inally, regional blood flow within the corpus luteum was studied using 15 µ radioactive microspheres. When small pieces of tissue were taken following infusions of HCG-1 2 5 I and 8 5Sr microspheres into the ovarian artery the cpm/mg tissue of the two isotopes were highly correlated. These data suggest that the specific binding of HCG to luteal cells is influenced by the flow of blood to these cells. From these studies it appears that blood flow may be an important factor in regulating the activity of the gonadotropic hormones at the luteal cell level. It further appears that a secondary mechanism of action of LH may be to increase blood flow to the corpus luteum.