OBJECTIVES: To measure heart rate (HR) and heart rate variability (HRV) at rest and during exercise in children with post-severe traumatic brain injury as compared to age-matched typically-developed controls. DESIGN: Comparative study. SETTING: Out-patient rehabilitation department. PARTICIPANTS: Twelve boys post-severe traumatic brain injury (TBI) (aged 7-13 years) and 18 typically-developed (TD) boys matched for age. INTERVENTIONS: HR and HRV were determined at rest and at a steady functional walking rate on the treadmill. MAIN OUTCOME MEASURES: HR and HRV parameters include: time domain parameters: standard deviation of the R-R interval, square root of the mean squared differences of successive R-R differences at rest and during steady-state exercise. RESULTS: Children post-TBI demonstrated higher mean HR values at rest (TBI 91.8 +/- 7.0 beats per minute vs 72.0 +/- 7.1 beats per minute in controls, p < 0.05) and during exercise (TBI 123.4 +/- 15.5 beats per minute vs 113.0 +/- 9.1 beats per minute in controls, p < 0.05). At rest, the time domain measures of HRV were significantly lower in the TBI group (p < 0.05). Time domain mean values decreased significantly during exercise only among the TD children. CONCLUSIONS: The findings of this study suggest that among children post-severe TBI, the cardiac autonomic mechanism is less efficient at rest and less adaptive to exercise and activity as compared to TD children.