Submaximal graded exercise testing is a commonly used method to assess cardiovascular stress. Reserve of repolarization of the heart (RoR) is an ECG-based, non-invasive method to monitor the heart’s stress response, and assesses cardiac cells ability to reestablish their membrane potential. RoR has been shown to be a useful indicator of cardiovascular disease risk in cardiac patients. This study examined RoR difference between trained (T) and untrained (U) individuals and correlate this outcome with blood stress markers. PURPOSE: To determine if RoR and blood stress markers in response to a graded exercise in T and U cohorts differ. METHODS: Thirty-nine (male and female) subjects (23.6 ±5.6 yrs) were recruited. Subjects arrived after overnight fast between 7-9 am and rested for 20 minutes. Subjects completed a fitness questionnaire to determine training status (T or U). ECGs (12 lead) were monitored before, during and after exercise to obtain RoR. A graded walking test (GXT) on a treadmill until 85% of estimated maximum heart rate was performed. Blood obtained at rest and immediately after exercise were analyzed using HPLC for glutathione (oxidized [GSSG], reduced [GSH], total [TGSH]). Repeated measures ANOVAs were utilized to analyze the results using SPSS v24 with significance set at α = 0.05. RESULTS: There were significantly lower resting HRs (p = .023) and higher workloads achieved during testing (p = .002) between T vs U groups. Resting RoR (independent of group) was significantly reduced from 75±5% pre-test to 26±10% RoR at the end of exercise (p < .001). Final stage RoR was significantly lower for T compared to U group (T: 20±9.4%; U: 31±9.4%, p = .041), but T group performed significantly greater stages (p=0.002). The GXT induced a reduction in blood GSH (Pre 366 ±161 μM, Post 273±180 μM, p < .001) and an increase of GSSG (Pre 135 ±62 μM, Post 157±83 μM, p = .038) with no difference between groups. GSSG/TGSH decreased after GXT (p=0.028) independent of group. CONCLUSIONS: These data suggest that a GXT induces a different stress response in T and U individuals. The oxidative stress at end GXT was similar but needed more workloads to get to this same internal stress level in the T group. Further studies are needed to ascertain stress responses with RoR and relative workloads.