The polypeptides in the present invention possess the effects of promoting and inhibiting the secretion of prolactin, and are thus useful as drugs for the prevention and treatment of various diseases, in terms of prolactin secretion stimulants, which are associated with the secretion of prolactin, such as hypoovarianism, spermatic underdevelopment, menopausal symptoms, hypothyroidism, etc. The polypeptides are useful as drugs for the prevention and treatment of various diseases, in terms of prolactin secretion inhibitors, which are associated with the secretion of prolactin, such as pituitary tumor, diencephalon tumor, menstrual disorder, autoimmune diseases, prolactinoma, sterility, impotence, amenorrhea, lactorrhea, acromegaly, Chiari-Frommel syndrome, Argonz-del Castilo syndrome, Forbes-Albright syndrome, lymphoma, Sheehan's syndrome, spermatogenesis disorder, etc.