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To examine cardiac autonomic modulation, via heart rate variability (HRV), after exercise bouts at 50% (LO) and 80% (HI) of O2 reserve (O2R).Thirteen male volunteers (age: 25.7 ± 3 yr) exercised on a treadmill at either LO or HI (randomly assigned) on separate days, expending 300 kcal during each bout. During each visit, electrocardiogram (ECG) recordings were obtained during a supine 5-min period before exercise (PRE) and during 30 min of postexercise supine recovery (POST). The last 25 min of POST was divided into five 5-min segments to closely examine postexercise HRV. ECG segments were analyzed for the standard deviation of RR intervals (SDNN), whereas total- (lnTP), low- (lnLF), and high-frequency (lnHF) power were obtained via spectral analysis. LF and HF were further normalized (LFNU and HFNU) to better quantify autonomic balance.There were main effects of time and intensity on SDNN such that it was lower than PRE until 15 min POST, and lower after HI. There were significant intensity × time interactions on lnTP, lnLF, lnHF, LFNU, HFNU, and lnLF/lnHF. Post hoc analysis revealed that lnTP and lnLF were lower than PRE up to 20 min POST after HI. lnHF was lower than PRE for 25 min POST for HI, and for 10 min POST for LO. LFNU and lnLF/lnHF were greater, whereas HFNU was lower, compared with PRE up to 15 min POST, and from 20–25 min POST for HI only.Exercise at 50% O2R caused less of a shift in the cardiac autonomic balance, with a quicker restoration of vagal modulation than exercise at 80% O2R.