There is still controversy regarding many of the problems that pertain to aggressive behavior. One of these issues is the classification of aggressive behavior. In the author's opinion, intraspecies intermale aggression can be classified as follows: (1) Stress- and fear-induced reactive defensive aggression; (2) Frustration-induced offensive aggression accompanied by anger; (3) Instrumental type of offensive aggression. (4) Aggression linked to pleasure. A number of problems related to the neuro- and biochemistry of aggression are also controversial. Many chemical agents participate in the induction of aggressive behavior. It is important to identify which agents take part in the realization of aggressive behavior in situ. Therefore, it is necessary to identify substances that simulate aggression which elicits adaptive responses to threatening stimuli in the environment. The authors hypothesize that the most suitable inducer of aggressive behavior must fulfill the following criteria: (i) when released in the organism or injected in physiological doses, it elicits/amplifies aggressive behavior; (ii) its synthesis and release increases during stress and agonistic encounters; (iii) its basal concentration in dominants is higher in brain structures responsible for aggressive behavior. Only norepinephrine fits all of the mentioned criteria. It stimulates an aggressive response and is produced actively when aggressive behavior is necessary for adaptation. (Published Abstract)