Exercise training benefits cardiovascular health by increasing cardiac vagal control. During recovery from exercise, cardiac autonomic control is disturbed and there is an increased risk of cardiac event. This study investigated changes in autonomic nervous system activity for 24 hours following a bout of dynamic exercise including the responses to a mental stressor (arithmetic test, MS). Following ethical approval, eleven (2 female) healthy subjects (age: 34.3±6.9years, height: 170.3±9.6cm, weight 72±11.2kg) gave informed consent to participate. Following maximal incremental cycle ergometer testing to determine exercise workload, each subject visited the laboratory on four occasions. Visit one and three: ECG was recorded at rest and during 5-min MS. Then also recorded for 75-min after 20 minutes of moderate exercise (ET) or bed rest (NET) with MS at 10-min and 50-min. Visit two and four: 24 hours after visit one and three respectively, including resting ECG and MS test. Two indices of vagal activity were calculated: the root mean square of successive differences in RR intervals (RMSSD) and high frequency (0.15-0.40 Hz) spectral power (HF). Two-way ANOVA with post-hoc paired t-tests between trials were used. No significant differences existed at baseline between trials. As expected, exercise significantly elevated HR (up to 75-min) and decreased vagal indices up to 50-min (p<0.05) but had returned to baseline values at 75-min. At baseline, MS increased HR significantly (8.4±6.5bpm) (p<0.001) and decreased HF (-100±1113.5ms²). At 24 hours, the HR response to MS was reduced by -3.9±4.5bpm following ET compared with NET (-3.3±3.7bpm). There was however, a trend to increase HF during MS (ET: 538.4±1169.1ms² Vs. NET: 178.3±781.2ms², p=0.096). Furthermore, at 24 hours, error rate in the MS answers were reduced in ET compared to NET (15.4% Vs. 20.4%, respectively). This indicates that vagal activity is reduced in ET up 50 minutes post-exercise but the increase in vagal activity and lower HR response at 24 hours post-exercise to MS suggests a cardioprotective effect against mental stress on the day following exercise.