Prolactin is a peptide hormone with an essential role in lactation. During breastfeeding, the act of an infant suckling the nipple stimulates the production of prolactin, which in turn stimulates the production of milk in a process called lactogenesis. Another hormone, oxytocin, is secreted in response to suckling to trigger milk let-down. In some animals, prolactin also influences seasonal changes in pelage, and is released in response to stress, and acts as a gonadotrophic hormone, in which capacity it has an important role in the maintenance of pregnancy in some mammals.
Prolactin is synthesised by lactotrope cells in the adenohypophysis (anterior pituitary gland). It is also produced, to a much lesser extent, in the breast, the decidua, parts of the central nervous system and the immune system.  The gene encoding prolactin in humans is located on chromosome 6. Like other hormones of the anterior pituitary, prolactin is secreted by a process of calcium-dependent exocytosis of the large dense-core vesicles that contain prolactin.
Pituitary prolactin secretion is regulated by neuroendocrine neurons in the hypothalamus, the most important ones being the neurosecretory tuberoinfundibulum neurons of the arcuate nucleus, which secrete dopamine to act on the dopamine-2 receptors (D2-R) of the lactotrophs, to inhibit prolactin secretion. Thyrotropin-releasing factor and oxytocin have a stimulatory effect on prolactin secretion. Prolactin itself acts in a negative-feedback manner on the brain.