Norway rats have been selected during 20 generations by the absence of aggressive reaction to man (tamed rats). From 7 up to 20th generations of selection, different forms of aggressive behaviour (reaction to glove, intermale, shock-induced aggression and predatory aggression) were studied, and the level of noradrenaline, serotonin and its metabolite 5-hydroxyindoleacetic acid was determined in the brain. In the absence of aggressive reaction to glove in tamed rats, the shock-induced aggression considerably decreased while the predatory aggressiveness (mouse-killing behaviour) and intermale aggressiveness did not change. Beginning from 15-16th generation of selection, a higher level of the 5-hydroxyindoleacetic acid in the hypothalamus was established, in the 20th generation an increased content of serotonin was revealed in the hypothalamus and the midbrain. In some generations of selection an increased level of noradrenaline in the hypothalamus in comparison to wild rats was observed. A conclusion is made that the selection of animals by taming unequally influences different kinds of aggressiveness and is accompanied by inherited consolidated reorganization of the monoamine brain systems.