Non-human primates naturally engage in prosocial behavior, but such behavior is inconsistently seen in experimental contexts. Two factors that may influence prosocial behavior are social relationships and the hormone, oxytocin. To investigate these factors in more socially relevant contexts, I gave four social groups of capuchin monkeys (Sapajus [Cebus] apella) an apparatus that one monkey could activate to provide juice to another in their group. High-ranking capuchins monopolized the apparatus, but they provided juice more often when a group member benefitted than in control situations in which juice was unavailable, suggesting that they understood the contingencies of the task. When activating the apparatus provided juice to another group member, capuchins pulled the lever more for close associates. Finally, endogenously released oxytocin increased donations to other group members in all conditions, suggesting that oxytocin made capuchins more likely to interact with the apparatus.