Rats, matched by weight and goal approach, were tested for aggressiveness using a food-competition paradigm. Those winning and those losing all of their matches were designated as high and low aggressive, respectively. A third group of randomly selected rats never fought and were used as a control for the fighting experience. Half of the rats were used to determine serotonin (5-HT) and 5-hydroxyindoleacetic acid (5-HIAA) levels in discrete brain regions. The other half were used to estimate 5-HT turnover in the same brain regions. High and low aggressive rats did not differ with regard to 5-HT or 5-HIAA levels, but high aggressive rats showed faster 5-HT turnover than low aggressive rats. The turnover differences reached statistical significance in only two of the 3 brain regions examined: (a) all structures (minus olfactory bulbs) rostral to the level of the anterior commissure, and (b) the brain stem (hypothalamus-midbrain-medulla). The observed differences cannot be attributed to differing behavioral experiences since the high and low aggressive groups pooled did not differ from the no-fighting control. These findings are discussed with respect to the involvement of serotonin in rat intraspecies aggression.