 Recent research has shown that hypoxia plays a crucial role in regulating ovary function during the ester cycle through the activation of hypoxia-inducible factor 1, HIF1, and its downstream target gene vascular endothelial growth factor, VEGF, which induces angiogenesis. Hypoxia also contributes to luteolysis by down-regulating progesterone synthesis and up-regulating apoptosis of luteal cells, and recent studies have focused on the roles of hypoxia and HIF1-regulated genes in bovine corpus luteum, CL, function. This article was authored by Ryo Nishimura and Kyoshi Okuda.