Early adverse care has been associated with a variety of negative developmental outcomes, including the development of psychopathologies and impairments in social competence and regulation of emotions. Concurrent alterations in neural threat- and stress-response systems may underlie maltreatment-related socioemotional and cognitive alterations observed in maltreated children and animal models. This study utilized an experimental, cross-fostering design to examine maltreatment-related alterations in social behavior, behavioral responses to a stressor, and synthesis and turnover of specific neuropeptides and neurotransmitters involved in systems associated with the stress response and psychopathologies. Subjects were 36 juvenile rhesus macaques (Macaca mulatta) cross-fostered at birth to either maltreating foster mothers (N = 18) or control foster mothers (N = 18). All data were collected when the subjects were 18 months of age. Undisturbed social behavior was measured via focal observations while the subjects were in their social group. Behavioral responses to a stressor were measured during an Approach/Avoidance test during which subjects were presented with a fear-evoking object. CSF samples were collected to measure concentrations of CRF, 5-HIAA, MHPG, and HVA. In the social group, maltreated subjects exhibited affiliative behavior toward juveniles significantly less than control subjects but were similar in other measures of behavior. During the Approach/Avoidance task, maltreated subjects spent a significantly smaller percentage of time than the control subjects visually inspecting the fear-evoking object potentially indicating that maltreated subjects were less vigilant than control subjects. Maltreated and control subjects were similar on other behavioral responses. CSF concentrations of CRF, MHPG, 5-HIAA, and HVA correlated with behavior observed in the social compound and during the Approach/Avoidance test but did not differ between maltreated and control subjects. Similarly, dimensions of early maternal care (i.e., protectiveness, responsiveness, security, irritability, average rates of abuse and rejection) correlated with behavioral measures with significant group differences, indicating that the early mother-infant relationship likely influences behavioral development later in life. These findings are discussed in terms of the developmental time course over which maltreatment-related alterations in behavior may appear, resilience, temperament, genetic heritability, and potential protective factors against the negative behavioral outcomes associated with experiencing early adverse care.