The concept of a major inhibitory role of serotonin in aggressive behavior is widely accepted by researchers. There were ample evidences that a pharmacologically-induced increase in serotonergic activity attenuates agonistic behavior, and the manipulations inhibiting the brain serotonergic system can elicit aggressiveness in animals. Ealier, experimental studies have demonstrated reduced metabolism in brain serotonergic system and specific changes in pharmacological sensitivity of 5-HT(1A) receptors in male mice with repeated experience of aggression. It has been shown that mRNA levels of the serotonergic genes in the midbrain raphe nuclei are reduced in aggressive males. After no-fight period expression of some genes is restored or becomes higher compared with the controls on the background of increased aggression. The review provides data supporting and contradicting the serotonin-deficiency hypothesis of increased aggressiveness, revealing non-specific role of serotonin in the control of aggressive behavior, and also provides information about the inhibition of serotonergic activity under repeated experience of aggression.