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In humans, and non-human primates, reunion following a separation results in a positive emotional state, and an increase in affiliative behaviors. To examine the role of opiate systems, in mothers and infants in mediating reunion behavior, morphine and naltrexone were administered after a brief separation. Infants administered morphine (0.1 mg/kg IM) showed a significant reduction in clinging and girning, a vocalization emitted during close physical contact. Naltrexone (5 mg/kg IM) had opposite effects. When administered to mothers, again morphine decreased and naltrexone increased clinging. Morphine administered to mothers had a more transient behavioral effect which could not be accounted of intimate contact between a mother and her infant is regulated by the reciprocal activation of their opiate systems. This activation of opiate systems may reinforce the infant's need for attachment and the mother's role …