During diestrus, the follicle has already ovulated and the corpus luteum is present on the ovary. The hypothalamus releases GnRH which stimulates LH release from the anterior pituitary. The LH then stimulates the corpus luteum to produce progesterone. A negative feedback loop is initiated in which the progesterone inhibits GnRH release from the hypothalamus, LH decreases and progesterone is maintained at a steady state. The progesterone is important in that it causes the diestrus behavior and an increase in uterine contractility referred to as uterine tone. At the end of diestrus, PGF2a is released from the uterus and causes regression of the corpus luteum.