Prolactin is a hormone that is secreted by the pituitary gland. Besides the pituitary origin, prolactin is found in other tissues and cells such as adipocytes, immune cells, skin, prostate, brain, and decidua. It is best known for its role in enabling female mammals to produce milk. However, prolactin also plays essential roles in the regulation of immune functions, pancreatic development, angiogenesis, reproduction, osmoregulation, and metabolism. This hormone is produced in increasing amounts during pregnancy and suckling, and it acts on the mammary gland by initiating and maintaining lactation in the postpartum period. Rat prolactin has 88.2% and 67.4% amino acid sequence homology with its mouse and human counterparts respectively. Epidemiological studies demonstrated that circulating high levels of prolactin increase the risk of breast cancer. Prolactin may promote mammary tumor initiation via the JAK2/STAT5 signaling pathway.