Introduction I n chuckwagon racing, dubbed the “half‐mile of hell”, teams often race multiple consecutive days. Horses are in peak condition, however rare cases of sudden death attributed to cardiovascular failure occur. There are concerns regarding cumulative racing and its impact on the risk of sudden death. The objective was to determine if cumulative exercise affects integrity of the myocardium, assessed by high‐sensitivity troponin‐T ( cTnT ), and if the presence of arrhythmias during race or recovery affects plasma cTnT concentrations. Methods Heart rate and electrical activity were recorded on 148 horses (9 ± 2.7 years) during racing days 1 and 2. 750 plasma samples were analysed (374 pre‐exercise, 376 post‐exercise) comparing cTnT before and after activity (paired t‐test). Associations between consecutive racing days, age and cTnT were analysed ( ANOVA , Pearson Correlation). The relationship between arrhythmias and cTnT was also examined. Results cTnT increased significantly with exercise (pre: 4.52 ng/ L ± 0.17, post: 5.20 ng/ L ± 0.23, P<0.001). However cTnt was not affected by cumulative exercise. A correlation between age and cTnT was found. 4235 arrhythmic events were recorded. No association existed between cTnT (post‐exercise) and the presence of arrhythmias during exercise and recovery. Conclusions This is the first study examining the effect of exercise on high‐sensitivity cTnT in horses and first large‐scale study on chuckwagon horses. Racing increases cTnT , however values remain low, suggesting myocardial damage of little clinical significance. Chuckwagon horses have long careers therefore age and cTnT could be further examined. cTnT was not affected by the presence of arrhythmias during exercise, indicating chuckwagon racing is well tolerated. Ethical Animal Research The study was approved by the Veterinary Sciences Animal Care Committee of the University of Calgary ( AC 120049) and onwer informed consent was obtained. Sources of funding: Clinical Research Fund from the University of Calgary's Faculty of Veterinary Medicine. S ummer Studentships were funded by NSERC . Competing interests: none.