Low levels of cardiac vagal tone have been shown to be a predictor of future cardiac dysfunction and mortality1. Post-exercise recovery is significant as strenuous physical activity is associated with an increased risk of cardiac events for a period of around 1 hour2, thought to be linked to poor return of vagal tone3. Delayed parasympathetic return and prolonged sympathetic activity following exercise may have important medical implications. However, improvements in cardiac autonomic control as a result of exercise training may attenuate the delay or imbalance1. This study aims to examine how endurance training in middle-aged subjects alters the autonomic recovery following acute exercise. Baseline ECG was recorded in 15 healthy human subjects (aged 36-49 years, 5 male, 10 female) (ADInstruments Powerlab) during paced breathing (0.25 Hz). Subjects then completed cycle ergometer exercise comprising of a 2 minute warm-up, 10 minutes at 75% age-predicted maximum HR (Karvonen formula), and 1 minute of recovery. Recordings were then taken at 0, 15, 30, 45, and 60 minutes post-exercise. This procedure was then repeated following 12 weeks of endurance exercise training (13 kilocalories/kg/week). Heart rate variability (HRV) and QT measures were assessed offline (Chart, ADInstruments). Changes from baseline were assessed by paired t-test. Values are presented as mean ±SD. Training improved the rate of recovery of HRV indices of cardiac vagal tone, with the root mean square of successive R-R interval differences (RMSSD) recovering to within baseline values (P>0.05) 30 mins post-exercise, whereas pre-training (PRE) time to recovery was 60 mins. High-frequency power took 45 mins to recover PRE, and 30 mins post-training (POST). HR remained elevated above baseline for 45 mins PRE (mean 69 ± 6.3 BPM at baseline, 73.5 ± 8 at 45 mins, p<0.05), and 60 mins POST (mean 65.3 ± 11 at baseline, 66.7 ± 11 at 60 mins, p<0.05), potentially demonstrating on-going and concomitant sympathetic activity. Markers of this are much more difficult to derive however. The low/high frequency power (LF/HF) ratio remains elevated (p<0.05) for 30 mins PRE, and 45 mins POST, suggestive of sympathetic activity. QT interval has been suggested to reflect cardiac sympathetic influences4. This remained significantly lower than baseline (p<0.05) at all time points PRE and POST, but only at 15 mins PRE and POST when corrected for heart rate using the Bazett formula. These data suggest that sympathetic activity remains elevated for a considerable period post-exercise, even when parasympathetic markers have recovered. This underlying sympathetic activity along with depressed recovery of vagal tone also may be contributing to the period of increased cardiac risk after exercise. However, neither LF/HF ratio nor QT interval appeared to provide any strong indication of this.