PURPOSE: This study investigated non-invasive indices of post-exercise parasympathetic reactivation (using heart rate variability, HRV) and sympathetic withdrawal (using systolic time intervals, STI) following different exercise durations. METHODS: 13 healthy males (age 26.4 +/- 4.7 years) cycled at 70% heart rate (HR) reserve for two durations-8 min (SHORT) and 32 min (LONG)-on separate occasions: HRV (including natural logarithm of root mean square of successive differences, Ln-RMSSD) and STI (including pre-ejection period, PEP) were assessed throughout 10 min seated recovery. RESULTS: Exercise HR was similar between SHORT and LONG (146 +/- 7 and 147 +/- 6 b min(-1), respectively; p = 0.173), as was HR deceleration during 10 min recovery (p = 0.199). HR remained elevated above baseline (p < 0.001) throughout recovery for both trials (SHORT 82 +/- 13 b min(-1); LONG 86 +/- 10 b min(-1), at 10 min post-exercise). Ln-RMSSD was similar at end-exercise between trials (SHORT 1.10 +/- 0.30 ms; LONG 1.05 +/- 0.73 ms; p = 0.656), though it recovered more rapidly following SHORT (p = 0.010), with differences apparent from 1 min (SHORT 2.29 +/- 1.08 ms; LONG 1.85 +/- 0.82 ms; p = 0.005) to 10 min post-exercise (SHORT 2.89 +/- 0.80 ms; LONG 2.46 +/- 0.70 ms; p = 0.007). Ln-RMSSD remained suppressed below baseline throughout recovery following both trials (p < 0.001). PEP was the same at end exercise for both trials (70 +/- 6 ms), with exercise duration having no effect on recovery (p = 0.659). By 10 min post-exercise, PEP increased to 130 +/- 21 ms (SHORT) and 131 +/- 20 ms (LONG), which was similar to baseline (p >/= 0.143). CONCLUSIONS: Prolonged exercise duration attenuated the recovery of HRV indices of parasympathetic reactivation, but did not influence STI indices of sympathetic withdrawal. Therefore, duration must be considered when investigating post-exercise HRV. Monitoring these measures simultaneously can provide insights not revealed by underlying HR or either measure alone.