Prolactin (PRL) is a peptide hormone secreted by the lactotroph of the anterior pituitary. Secretion of PRL is regulated negatively by hypothalamic hormone. Dopamine is the major physiologic prolactin inhibiting factor (PIF). PRFs as TRH, VIP and oxytosin have prolactin releasing activity. Prolactin secretion is induced by sleep, stress, sexual intercourse and suckling stimulus. Serum level of PRL is increased by estrogen. Therefore, PRL level is higher in women, especially in the late follicular phase and during pregnancy. Lactation is the major biologic function of PRL in humans and essential to reproduction. Other function of PRL, such as regulation of ovarian function, osmoregulation and immunoregulation is not fully established. Many action of PRL remained to be clarified.