Sixteen squirrel monkey mother and infant dyads were housed in groups of two. They were separated and then immediately reunited in their familiar home cage or in a novel environment, either in conjunction with the second mother-infant dyad, or as a single manipulated pair. Behavioral observations of both dyads were made during the 30-min period following these manipulations; blood samples were then taken for cortisol assay. Mothers' locomotor activity increased when a single dyad was manipulated in the home cage and decreased in the novel environment where proximity between the pairs increased. Cortisol levels varied significantly across conditions, with mothers and infants responding to different stimuli. Mothers responded primarily to social disruption, including separation from the other dyad, and infants responded most clearly to novelty and separation from the other dyad. No significant differences were observed over base levels when both mother and infant pairs were reunited in the home cage, indicating that changes were not due to the disturbance involved in the separation procedure.