Objective: The objective of this study was to determine whether seated cervical manipulation produced changes in autonomic nervous system activity, as measured by heart rate variability and plasma norepinephrine levels. Methods: Ninety-five healthy young adults (ages 20-48 years) were recruited into a single-blinded physiological study, with 47 randomized to a seated cervical manipulation and 44 randomized to a sham procedure. Heart rate variability in the frequency domain, and plasma norepinephrine levels were measured prior to, immediately following, and 5 minutes following the intervention. Results: Electrocardiograms were obtained from 39 subjects in the sham group and 43 subjects in the manipulation group. No statistically significant changes were found in measures of heart rate variability in the frequency domain in either the manipulation or sham groups. Blood samples were obtained from 22 subjects in the sham group and 27 subjects in the manipulation group. Plasma norepinephrine levels, as measured by spectrophotometry, declined in both groups from pre- to immediately postintervention, and they remained at decreased levels 5 minutes after the interventions. There were no statistically significant differences between groups in pre- or postintervention norepinephrine levels. Conclusions: Measures of heart rate variability and plasma norepinephrine levels did not show that seated cervical manipulation produced short-term changes in autonomic nervous system activity compared to a sham procedure in healthy young adults. Keywords: Autonomic Nervous System; EKG; Norepinephrine; Spinal Manipulation.