The objective of the current research was to study the large individual differences in alcohol effects on aggressive behavior under systematically varied conditions in experimental protocols with mice. Three experiments were conducted with outbred Swiss-Webster derived mice that identified those individuals whose aggressive behavior was reliably heightened by low acute alcohol doses. In all experimental protocols, low alcohol doses were orally administered to a "resident" male mouse that subsequently confronted an "intruder" opponent for 5 min while all salient elements of aggressive behavior and motor activities were quantified. In all three experiments, alcohol (1.0 g/kg) heightened aggressive behavior by at least two standard deviations of the individual's water vehicle control mean in 27% of the mice. In 64% of mice, no reliable change in aggressive behavior was detected after the identical alcohol treatment, and in 9% of the mice alcohol decreased aggressive behavior. Experiments differed in protocol indicating that these aggression-heightening effects were evident in resident mice that were either maintained at restricted or unlimited amounts of food, housed singly or in breeding pairs with a female partner, and conditioned to perform daily a food-reinforced task or remained undisturbed. The first experiment found the aggression-heightening effects to persist during weekly challenges for at least 2 months (n = 8 of 30). The second experiment showed these effects at intervals from 5 to 60 min after alcohol administration. Blood alcohol concentrations reached peak level within 5 to 10 min after oral administration in mice that had confronted an intruder. Those mice in whom alcohol heightened aggressive behavior (n = 21) did not differ from those that showed suppressed levels (n = 9) in terms of blood alcohol concentrations (79.6 vs. 82.4 mg%), suggesting that the intensity and frequency of aggressive behavior after alcohol were not directly dependent on the amount of alcohol in the circulation. The third experiment revealed that alcohol's (0.1 to 5.6 g/kg) effects on heightened aggressive behavior (n = 11) are dissociated from those on concurrently measured high- or low-rate operant performance as engendered by a multiple FR 30-FI 600 sec schedule of reinforcement. Current results indicate that this alcohol effect is relatively specific to aggressive behavior in individual animals, offering the opportunity for neuropharmacological and molecular characterization.