Research on psychophysiological functioning in early onset psychosis is reviewed within the context of etiological formulations stressing perceptual and selective attention abnormalities. Electroencephalo-graphic (EEG) measures, including resting EEG level, alpha blocking, average evoked response, and contingent negative variation, have received most attention in the literature; cardiovascular and electrodermal indices have not been so extensively explored in this group of children. While the nature of differential reactivity in early onset psychosis has not been thoroughly investigated, limited evidence suggests that the central behavioral anomalies associated with the condition may result from abnormal neurophysiological reactivity to sensory stimulation. Further studies of the psychophysiological functioning of these children should yield data useful in the clarification of etiology.