In previous studies we reported that chronic administration of theophylline induced affective aggressive behavior in rats. The present study was designed to clarify characteristics of the aggressive behavior observed, and compare it with that in the olfactory bulbectomized and isolated rats. To measure aggressive behavior, the reflexive fighting test, the test for killing response to various objects and the aggressiveness rating test were used, and in addition, the open field test was employed to objectively measure emotionality. It was only the theophyllinized rats that assumed the fighting position not only during the electric shock period but also during the adaptation and sound shock periods. The theophyllinized pairs fought quicker and more frequently than did the other treated pairs. During a 3 min observation the percentage of mouse-killers was much higher among the theophyllinized group (62.5%) than among the other treated groups (the olfactory bulbectomized and the isolated groups increased mouse killing to 87.8% and 62.5%, respectively, though the theophyllinized group remained at 62.5%. The theophyllinized killers attacked constantly rat-pups and dead as well as live mice. In the open field test, rearing and ambulatory activities increased more in the olfactory bulbectomized group and decreased more in the isolated group than in controls, but no change was observed in the theophyllinized group. Defecation which has a negative correlation with aggressiveness decreased more only in the theophyllinized group than in the controls. Prior to treatment there was a positive correlation between the frequency of ambulation and that of rearing (r = 0.535, n=32, p less than 0.01).