Adult male Swiss-Webster mice were housed either singly (isolated) or with a female (nonisolated). Aggressive behavior was evoked by introducing a group-housed male mouse (intruder) into the home cage of the isolated or nonisolated mouse (resident).d-Amphetamine, methamphetamine, methylphenidate, cocaine, and L-dopa decreased attack and threat behavior by resident mice, the isolates requiring 2--4 times higher drug doses for the antiaggressive effects than the nonisolates, d-Amphetamine, methamphetamine, and methylphenidate caused intruder mice to be more frequently attacked by their non-treated resident opponents, to escape more often, to assume the defensive upright posture less, and to move about more often. L-Dopa nonspecifically de creased all elements of agonistic and nonagonistic behavior, while the amphetamines and methylphenidate suppressed attacks, increased escapes, decreased upright postures, and increased nonagonistic locomotion. By contrast, cocaine's antiaggressive effects were remarkably specific, i.e., not accompanied by changes in other behavioral elements.