Purpose <p>Sympathetic nervous system (SNS) functioning in autism is of interest etiologically and clinically. It remains unclear whether there is a chronic state of hyperarousal or a heightened physiological response to stressful situations, or both, in autism (ASD). A systematic review of neurochemical studies of SNS functioning in ASD addressed these issues.</p> Methods <p>The PubMed, Scopus, Ovid, and Web of Science databases were searched using a first set of search terms consisting of norepinephrine (NE), epinephrine (EPI) and their known metabolites. A second set of terms consisted of relevant diagnostic terms. The sets were combined, and the retrieved studies were screened by three independent reviewers.</p> Results <p>With one exception, there were no significant differences between groups for the urine measures and for all plasma metabolite measures. However, plasma EPI showed a significant elevation in the ASD group and plasma NE showed a trend level increase. The reported variances were similar in the two groups, and typical nighttime decreases in sympathetic measures were reported in ASD groups.</p> Conclusions <p>The results indicate that mean basal SNS functioning is not increased in ASD. In contrast, the plasma EPI and NE findings are consistent with increased reactivity to acute stressors in ASD. There was no indication of an admixture of hyper- and hypo-arousal subgroups and typical nighttime sympathetic diminution was seen in ASD. It appears that sympathetic functioning is not atypical in ASD and that the apparent heightened reactivity to stress is more likely due to increased elicitation of the sympathetic system in ASD.</p>