The present study examined plasma cortisol and behavioral responses to environmental novelty in squirrel monkey and titi monkey male-female pairs. Overall, seemingly trivial increments in novelty evoked sustained plasma cortisol elevations. In individually tested animals, the minimal level of novelty sufficient to evoke a cortisol response was smaller, and the ability of the response to discriminate among levels of novelty was greater, in titis than in squirrel monkeys. When tested with the pairmate, the sensitivity of the response was reduced in titis but not in squirrel monkeys. Behavioral measures were not as sensitive to novelty as was the cortisol response. The results suggest that differential endocrine responsiveness to novelty is an important physiological concomitant to previously described differences between squirrel monkeys and titi monkeys in their characteristic modes of relating to the environment.