Analyzes of cardiac autonomic responses at the initial transient of exercise have been used for the investigation of the cardiovascular health. We evaluated the influence of aerobic fitness on HR and HRV responses at the onset of exercise. 25 male subjects (22.3+/-2.4 years) were divided into 2 groups: \'low aerobic fitness\' (36.2+/-2.6ml.kg(-1).min(-1); n=10) and \'high aerobic fitness\' (46.4+/-5.0ml.kg(-1).min(-1); n=15). The experimental session consisted of assessing the beat-to-beat HR at rest and during submaximal exercise. The autonomic responses at the onset of exercise were calculated by fitting the HR and HRV (rMSSD-index) curves during the initial 300s of exercise into a first-order exponential equation. The time constant of HR and of the rMSSD index (tauonHR and tauonrMSSD) were calculated for analysis. We observed lower values of tauonrMSSD in the high aerobic fitness group compared to the low aerobic fitness group (26.8+/-5s vs. 38.0+/-18s, respectively; p=0.02). The tauonHR (42.0+/-15 vs. 49.3+/-26s, p=0.38) for the groups showed no difference. Aerobic fitness partially influenced the autonomic responses during exercise, since individuals with higher fitness showed faster decreases in beat-to-beat HRV at the onset of exercise.