Progesterone (10 mg) administered intramuscularly induces a concurrent release of prolactin as well as gonadotropin in estrogen-primed women. The time course of pituitary release of all three of these hormones appears to include a latent phase of 4 hrs and is maintained for at least 5 hrs. It is considered that this effect of progesterone may be mediated through a reduction of hypothalamic dopamine.