In Experiment 1, rat colonies consisting of two males and one female were established when the animals were either 100, 200, or 300 days old. All colony members were reared from weaning in small, stable isosexual groups prior to colony formation. Males that were 200 days old engaged in significantly more fighting at colony formation, and, three weeks later, were more likely to wound an intruder during a 24 hr aggression test than either younger or older animals. These findings demonstrate that colony aggression in male rats varies markedly as a function of age when reared under housing conditions that are employed in most laboratories. In Experiment 2, colony females were individually housed after their second pregnancy and their response to a male intruder was evaluated for a 24 hr period after nine days of lactation. Maternal aggression occurred at all ages, indicating that age may be of lesser importance in nest defense of females than intermale conflict. However, maternal aggression was ineffective in preventing the destruction of a high proportion of the litters by intruders in the three maternal age groups.