Phasic changes in respiratory period, electrodermal activity (EDA), the evoked cardiac response (ECR), and the vasoconstrictive pheripheral pulse amplitude response (PPAR), were examined in matched groups of autistic, retarded, and normal children, using repeated presentation of simple visual and auditory stimuli. Analysis as a function of group membership and age indicated that respiratory pause and EDA showed habituation in the retarded and normal groups, but not in the autistic group, with no age effects in any group. PPAR and ECR showed no habituation in any group, but within-group age differences, and a higher mean response level for the autistic subjects. The autistic children did not display the diminution of response level with increased age that was characteristic of both the control groups. These effects were obtained across both modalities. Results could not be attributed to between-group tonic cardiac arousal differences, and were interpreted as further indications of diminished sensitivity to reductions in stimulus novelty, as well as developmental delay, in early-onset psychosis. © 1984.