This study tested the responsiveness of adult male squirrel monkeys living with mother-infant dyads to various conditions of mother-infant separation. During each of the five weekly experimental 30-minute sessions, as well as during undisturbed basal conditions, the behavioral and hormonal responses of five adult males were assessed. The results indicated that only one experimental condition-that involving the actual manipulation of the separated infant during the test session-produced behavioral and adrenocorticoid changes. During this condition the males exhibited increased movement, increased social activity scores, and elevations in plasma cortisol. These data indicate that squirrel adult male squirrel monkeys respond to mother-infant separation when this manipulation involves threat to the infant, as has been noted for other primate species.