Milk delivery through suckling lowers human newborn heart and metabolic rats. In rat and human infants pain threshold is markedly elevated and crying is arrested. These changes are induced by milk flavor or sugar taste which release central endorphins. In rats the changes are naloxone reversible; human infants born to women who were maintained on methadone during pregnancy were not quieted by sweet taste. Cholecystokinin (CCK) released during milk absorption quiets infant rats as does beta-casomorphine derived from casein hydrolysis. Through opioid and CCK mediation, milk also causes affective change that facilitates infant-mother bonding.