Prolactin (PRL) is a hormone released by the pituitary gland that is responsible for lactation (the production of breast milk in females). It is present in both males and females and is responsible for other functions in addition to lactation. In fish prolactin is used for osmoregulation which is the maintenance and regulation of water salinity inside and outside of the fish. In humans high levels of prolactin can cause a decrease in the levels of sex hormones (estrogen in women and testosterone in men). Prolactin also promotes neural growth in maternal and fetal brains.