AIM: To assess heart rate (HR) and heart rate variability (HRV) at rest, during exercise and during the recovery phase post-exercise in children at the chronic phase post-severe TBI as compared to age-matched typically-developed (TD) controls. SETTING: Out-patient clinic. PARTICIPANTS: Ten children (two girls, eight boys), 3-5 years post-severe TBI, aged 7-11 years with residual deficits and 20 TD children matched for age. INTERVENTIONS: HR and HRV were determined at rest, during step test for 3 minutes, during walking on a treadmill for 6 minutes and during the recovery periods post-exercise sessions. MAIN OUTCOME MEASURES: HR and HRV parameters. RESULTS: Children post-TBI demonstrated higher mean HR values and lower HRV at rest compared to controls (p < 0.05). During exercise a significant increase in HR and significant decrease in HRV was noted in both groups. A significant interaction was noted (p < 0.01); HR and HRV parameters in response to exercise and to exercise cessation were significantly lower among children post-TBI as compared to the controls. CONCLUSIONS: The findings of this study show that, in children, post-severe TBI at the chronic phase, the cardiac autonomic system is less efficient at rest and less adaptive to exercise and activity as compared to TD children.