The hypothesis was tested that one of the critical mechanisms underlying genetically determined aggressiveness involves brain serotonin 5-HT(1A)-receptors. The expression of 5-HT(1A)-receptor mRNA in brain structures and functional correlate for 5-HT(1A)-receptors identified as 8-OH-DPAT-induced hypothermia were studied in Norway rats bred over the course of 59 generations for the low and high affective (defensive) aggressiveness with respect to man and in highly aggressive (offensive) MAO A-knockout mice (Tg8 strain). Considerable differences between the aggressive and the nonaggressive animals were shown. Agonist of 5-HT(1A)-receptor 8-OH-DPAT (0.5 mg/kg for rats and 2.0 mg/kg for mice, i.p.) produced a distinct hypothermic reaction in nonaggressive rats and mice and did not affect significantly the body temperature in aggressive animals. In aggressive rats, a significant reduction of the expression of 5-HT(1A)-receptor mRNA was found in the midbrain. In Tg8 mice, 5-HT(1A)-receptor mRNA level was increased in the frontal cortex and amygdala and not changed in the hypothalamus and the midbrain. The results provide support for the idea that brain 5-HT(1A)-receptors contribute to the genetically determined individual differences in aggressiveness.