Autistic children's blood pressure, heart rate, peripheral blood flow (PBF), and peripheral vascular resistance (PVR) were measured during three types of tasks (reaction time, social interaction, at rest). The most highly functioning autistic children who learned the reaction time task to a criterion level had PBF measurements consistent with sensory intake during task performance and social interaction. The lower functioning children who failed to learn the task, even after 25 training sessions, showed PBF patterns consistent with rejection of external sensory information. When transferred from school to laboratory, the more impaired children showed greater response disruption and an increased heart rate, consistent with their behavioral need to preserve sameness. Autistic children appear to be more sensitive to the environment and may use behavioral strategies, such as avoiding environmental change and social interaction, as methods of reducing further disorganizing experiences. Disturbances in filtering environmental stimulation and modulating response to novelty may be part of the basic pathology of autism apparent during the 1st year of life.