Phasic changes in respiratory period, electrodermal activity, and the vasoconstrictive peripheral pulse amplitude response, were examined in matched groups of autistic, retarded, and normal children using repeated presentation of simple visual and auditory stimuli of differing magnitudes. Analysis of response magnitudes as a function of group membership, trials, stimulus magnitude, and age, indicated both similarities and differences between the autistic and control groups. The autistic group differed from the control groups in its failure to show response habituation to repeatedly presented stimuli. However, data indicated that autistic children coded stimulus magnitude similarly to controls, suggesting that the failure to adequately process stimulus novelty does not reflect a general processing failure. Autistic children also exhibited relative hyperreactivity in all measures. Age effects showed this to be interpretable as reflecting developmental delay. These two differences support a recent two-factor theory of autism. © 1988.