The relevance of central neurotransmission to aggressive and impulsive behavior has become more evident due to extensive research in humans and in animals. Among other findings, there are abundant data relating low serotonergic activity – as measured by low cerebrospinal fluid 5-hydroxyindolacetic acid, and a blunted response of prolactin to fenfluramine – to impulsive behavior. Many studies on testosterone activity show a relation between high plasma levels and a tendency towards aggression. It is hypothesized that the interaction between low serotonin and high testosterone levels in the central nervous system has a significant effect on the neural mechanisms involved in the expression of aggressive behavior. It seems that testosterone modulates serotonergic receptors activity in a way that directly affects aggression, fear and anxiety. Our survey reviews the main findings on serotonin, testosterone and the possible interaction between them with regard to these behavioral phenomena.