Defensive behaviors motivated by fear protect organism from danger; but intense fearfulness or activation of inappropriate defensive behaviors can result in behavioral, social, and physiological dysfunction. In young rhesus monkeys we have characterized three patterns of defensive behavior, discovered the environmental cues that elicit them, and developed a model for studying their differential regulation. Preliminary studies suggest that expression of these behaviors is regulated by opiate, benzodiazepine, and CRH systems. The objective of the proposed studies is to determine the developmental time course of infant monkeys' defensive behavioral and endocrine responses and to examine the neurochemical regulation of their expression. Our paradigm elicits defensive responses under 3 conditions: ALONE (infants briefly separated from their mothers, resulting in a high rate of species- species distress calls), and two conditions involving environmental threat. In NO EYE CONTACT, infants are separated from their mothers and exposed to a videotaped human face in profile, resulting in behavioral inhibition and reduced distress calls. In STARE, separated infants are exposed to a videotape of the same face neutrally looking directly ahead, resulting in barking and aggressive gestures. The studies are in three parts. Part I examines the ontogeny of the rhesus monkey's behavioral and endocrine defensive responses. This will establish the age at which infants away from their mothers can discriminate relevant threatening stimuli and express adaptive defensive responses. Part II tests whether prior experience with the threatening situation modulates behavioral and endocrine responses. Part III utilizes dose-response studies with selective agents to assess the roles of opiate, benzodiazepine, and CRH systems in regulating defensive behaviors and endocrine responses. Our model is highly relevant to human emotional development and fear- related behavior. It lets us examine the developmental and neurochemical factors mediating appropriate expression of defensive behaviors in primates. Understanding these behaviors may provide insight into mechanisms involved in the onset of stranger anxiety, temperamental differences in personality development, and development of anxiety-related psychopathology.