AbstractThis experiment was designed to determine whether isolation during two developmental periods would contribute to differences in home‐cage agonistic behavior and whether altered reactivity was a mediating variable. While early isolation (16–41 days) was shown to have a significant and sustained impact on agonistic behavior, isolation during a later developmental period (41–68 days) did not result in altered occurrences of agonistic behavior. While isolation did result in increased reactivity to both footshock and dorsal tactile stimuli, the pattern of these data suggested that hyperreactivity to tactile stimulation was not a satisfactory account of the increased agonistic behavior of rats raised in isol