- Researchers have recently hypothesized that autism spectrum disorders (ASD) may be partly characterized by physiological over-arousal. One way to assess physiological arousal is through autonomic measures. Here heart period (HP) and parasympathetic activity measured by respiratory sinus arrhythmia (RSA) were examined in adults with ASD and matched controls at rest and during performance of an emotional Stroop task. Resting HP and RSA were lower in adults with ASD than in matched controls, consistent with hypothesized over-arousal in ASD. However, dividing the ASD group on the basis of antipsychotic medication usage revealed that group differences in autonomic arousal may be related to the effects of these medications or their correlates. Autonomic adjustments for Stroop performance were comparable across groups, but in the control group, larger RSA reductions were correlated with faster responding (i.e., better performance). This relation was reversed in the unmedicated ASD group and absent in the medicated ASD group. Findings highlight the importance of considering medication status in the recently burgeoning area of psychophysiological studies of autism.