Based on previous research, the author identifies three subfactors of impulsiveness: motor impulsiveness, which involves acting without thinking; cognitive impulsiveness, which involves making quick cognitive decisions; and Fnonplanning impulsiveness, which involves a lack of concern for the future. Impulsive aggression is noted by the author to be characterized by a "hair- trigger" temper; that is, the impulsive-aggressive person usually responds aggressively without thinking and, following the aggressive act, often expresses guilt and remorse, vows not to commit the act again, but lacks the self-control to refrain from doing it again. The author proposes that the personality trait of anger-hostility is related to impulsive aggression. In a pilot study of the effect of phenytoin on impulsive aggression among inmates in a maximum-security prison, phenytoin significantly reduced the frequency of impulsive aggressive acts. In a more extensive study in progress, the results to date confirm the results of the exploratory study, with 7 out of 10 persons having fewer impulsive aggressive acts when taking phenytoin compared to the placebo condition. 2 tables and 69 references