In a lactating mammal, the two hormones that promote milk synthesis and milk release, respectively, are prolactin and oxytocin.
Prolactin is necessary for milk synthesis to occur. Ther are prolactin receptors on the walls of the lactocytes (milk-producing cells of the alveoli) which allows prolactin to stimulate the synthesis of breastmilk components. Oxytocin (after birth) induce contractions of the smooth muscles that surround the alveoli. As a result, the produced milk is squeezed into the duct system. Oxytocin is necessary for the milk ejection reflex in response to suckling.