Radioimmunoassayable prolactin rises in postpartum women during nursing and after intravenous thyrotropin-releasing hormone (TRH). Prolactin release induced by TRH can be dissociated from the postsuckling response. In addition to this, increases in endogenous prolactin secretion are followed by marked breast engorgement and milk letdown, especially after intravenous TRH. In this group of breast-feeding women, vaginal smears remained atrophic even up to 410 postpartum days. Prolactin appears to influence the production of breast milk, and the maintenance of a regular nursing pattern seems to promote the maintenance of ovarian unresponsiveness to circulating gonadotropins.