Evoked potentials (EP) which developed in the Cz point in response to 80 db tones (orienting response, OR) and to 105 db tones (defensive response, DR) as well as their dynamics in the process of extinction have been studied in 86 subjects (monozygotic and dizygotic twins). Distribution of the phenotypical dispersion into the genotypical and environmental components has been performed by means of the modified bio-metrical analysis. The genotype predominantly influences variation of amplitudes of the PI and P2 components and to a lesser extent of the P3 component.