Such effects were studied in male albino mice maintained under isolation circumstances for 7 weeks in order to induce aggressiveness. L-DOPA (25 mg/kg) was given concomitantly with DDC (75 mg/kg) or reserpine (0.1 mg/kg), or each was administered signly and intraperitoneally to subjects twice weekly. Two peaks on the aggressive degree were observed at the 3rd-4th and 6th weeks, respectively. At the first peak, mice treated with L-DOPA and/or reserpine demonstrated aggressive behavior to a higher degree than control mice but at the second peak, to a lower degree. Mice treated with L-DOPA and DDC showed the highest degree at the second peak. Tyramine uptake in the brain measured at the 6th week was enhanced in mice treated with combinations of L-DOPA with reserpine or DDC. Noradrenaline content in the brain was lowered in mice treated with L-DOPA and/or reserpine, in comparison with each control value; It is thus concluded that catecholamine-related drugs influence the degree of enhancement of aggressiveness in modes which vary depending on the form of action of each drug.