Prolactin is a polypeptide secreted by the lactotropic cells of the anterior pituitary. It initiates and maintains lactation and development of the mammary gland. There are several circulating forms: monomers, dimers, aggregates and glycosylated forms. Its serum levels exhibit remarkable diurnal rhythm. The determination of prolactin levels is helpful for the diagnosis of hyperprolactinemia, which disbalances the gonadal function at both sexes. The determination is also used for following-up of prolactinoma and other pituitary tumours.