SummaryProlactin is a hormone important for the functions of reproduction and has also general metabolic actions. By its action as luteotrophin, the hormone maintains the development and function of the corpus luteum. It has a fundamental role in producing the mammary gland development and milk secretion in mammals, the milk crop secretion in pigeons and doves and the mucin oviduct secretion of toads that maintains eggs for their fecondation. In some cases, prolactin has antigonadal action, probably by pituitary inhibition. Prolactin can awake maternal behaviour in some animals. In these functions, prolactin can act alone and more frequently in association with or conditioned by other hormones.Prolactin and progesterone have a role in the progestational preparation of the endometrium, in the maintenance of pregnancy and in the provision of food for the offspring after delivery.The general metabolic actions are different depending on the species. In birds, prolactin produces a great increase in food ingestion, in body weight and splanchnomegaly (liver, intestine, pancreas and crop gland), increase of respiratory exchanges and some modifications in fat metabolism. In some cases, prolactin has an effect on certain growth phenomena (enlargement of the epiphyseal cartilage of the hypophysectomized male rat, increase of body and visceral weight in pigeons, ...).Prolactin has an action on carbohydrate metabolism. It can develop diabetes in dogs or cats with previous large pancreatectomy. The three diabetogenic hormones of the pituitary gland are : somatotrophin, prolactin and adre- nocorticotrophin. In many cases, prolactin action is conditioned, reinforced or modified by other hormones : sexual, adrenal or hypophyseal. Probably, most of the physiological roles of prolactin are exerted in relation with these hormones, in the regulation of functions that depend on an endocrine equilibrium, in which the action of prolactin is dominant (development of mammary gland, milk secretion, crop milk secretion, etc.) or coadjuvant.