Habituation of evoked potentials and of autonomic responses to indifferent and unpleasant stimuli has been studied in 86 subjects. It has been found out that the property of unpleasantness as well as manifest defensive reactions significantly change the pattern of habituation. It is suggested that besides a well-known mechanism of habituation leading to a reorganization of "the neural model of the stimulus" there exist another mechanism responsible for changes in subjective qualities of the stimulus corresponding to the already available neural model. This second mechanism becomes activated in some people as a result of unpleasant stimulation which may exert significant influence on the dynamics of a number of psychophysiological responses and on the verbal assessment of the stimulus.