We introduce our recent approach to study autonomic nervous system control of heart rate during exercise by means of heart rate variability (HRV) spectral analysis with special reference to its relationship to ventilatory threshold (Tvent). The rationale for the study was that HRV has been shown to reflect (cardiac) parasympathetic and sympathetic nervous system (PNS and SNS, respectively) activity, together with the underlying complexity of cerebral autonomic system in terms of fractal dimension (DF) of HRV time series. The experimental results showed that PNS was markedly reduced below Tvent, that the rate of change in sympathoadrenal activity indicators (plasma norepinephrine and epinephrine concentrations and SNS indicator) was enhanced above Tvent, and that these changes in PNS and SNS indicators were associated with the appearance of the low-dimensional (low DF) dynamics that might reflect less complex autonomic activity. These findings have been considered with respect to implication for clinical cardiology.