The tendency to ''augment'' or to ''reduce'' the amplitude of the response to stimuli of increasing intensity is a matter of much interest in the neurophysiological, psychophysiological and psychiatric literature. According to their response, individuals have been divided so far into ''augmenters'' and ''reducers'', and different mechanisms beyond these types of response have been discussed. In most studies, however, the major emphasis has been laid upon ''reducing'', which has been regarded as deviance from the expected ''augmenting''. In the present article, a new theory referring to a two-directional adaptive switch mechanism for coping with incoming stimuli is presented together with some hypotheses which can be derived from such a general theory. In the following, evidence from experimental work is presented which supports the main theoretical principles, and also the hypothesis that both basic characteristics of the central nervous system and biochemical variables contribute to the determination of the individual augmenting/reducing response.