Male deer mice (Peromyscus maniculatus bairdii) were reared in a long photoperiod and housed individually from 3 weeks of age until they were killed 2, 4, or 6 weeks later. Males that were exposed to aggressive females for 2 min, three times per week, were of normal body weight and healthy, but had smaller reproductive organs than did males exposed to a control treatment. Reproductive development in male deer mice is responsive to at least two classes of social stimuli: pheromones and aggressive behavior.