PURPOSE: To examine whether differences in heart rate variability (HRV) can distinguish sub- from supra-ventilatory-threshold exercise and whether the exercise duration at supra-threshold intensity alters cardiorespiratory synchronization. METHODS: Beat-to-beat RR interval, VO2, VCO2, VE, and blood lactate concentration of 11 healthy well-trained young subjects were collected during two exercise tests: 1) a moderate-intensity test: 15 min performed below the power at ventilatory threshold (pVT); and 2) a heavy-intensity test: above pVT until exhaustion. Fast Fourier transform, smoothed pseudo Wigner-Ville distribution, and complex demodulation were applied to RR time series. RESULTS: 1) Moderate exercise shows a prevalence of low-frequency (LF) spectral energy compared with the high-frequency (HF) one (LF = 80 +/- 10% vs HF = 20 +/- 10%, P < 0.001), whereas the reverse is observed during heavy exercise (LF = 11 +/- 8% vs HF = 89 +/- 8%, P < 0.001). 2) During heavy exercise, the HF amplitude and the tidal volume (Vt) remained constant, whereas the breathing frequency (BF) increased (BF: 0.70 +/- 0.18 vs 0.93 +/- 0.31, P < 0.01) and mean RR decreased (342 +/- 15 vs 317 +/- 16, P < 0.01). Despite the RR series and the breathing signal remaining synchronized, HR/BF ratio decreased and stabilized at 3 RR for one breathing cycle, whatever the initial ratio. CONCLUSION: 1) HRV allows us to differentiate sub- from supra-ventilatory-threshold exercise and 2) exercise duration at supra-threshold intensity does not alter the cardiorespiratory synchronization.