Dietary amino acid regimens designed to enhance catecholaminergic and serotonergic functioning were found to differentially affect territorial-induced attacks in mice. Male albino mice were maintained on a semi-synthetic 12% casein protein diet for 2 weeks, then switched to diets modified by the addition of a 4% L-amino acid supplement, or 4% casein (control). Measures of aggressive behavior and open-field locomotor activity were obtained before and after the dietary supplements were administered. Resident mice fed supplements of L-tyrosine displayed a marked increase in the number of attacks on intruders and shorter attack latencies, but their locomotor activity was unaffected. L-phenylalanine supplements alone or in combination with L-tyrosine reduced the latency to attack and increased motility but did not affect the number of attacks. As a whole, the group of animals fed L-tryptophan showed no changes in aggression or motility.