It is now very clear that apparently healthy racehorses can have cardiac rhythm disturbances during exercise. Equally, arrhythmias can be a cause of poor performance. The challenge for the clinician is determining whether or not a rhythm disturbance is clinically relevant. Post-exercise cardiac rhythm is readily evaluated with smartphone ECG devices and there are a range of options available for recording the ECG during exercise. Ideally, for diagnostic purposes, exercise ECG should be performed at, or above the workload the horse is expected to perform. If there is a high index of suspicion, then multiple tests may be required. Rhythm disturbances which are likely to be clinically relevant include atrial fibrillation, ventricular bigeminy/trigeminy, polymorphic rhythms, R-on-T and where there are runs. Transient episodes of ventricular rhythms immediately after peak exercise are of uncertain significance, and the clinician should bear in mind that concurrent problems such as dynamic airway obstructions could be contributing to arrhythmiogenesis. Irregular rhythms which are present at both exercise and rest are more likely to relate to structural myocardial pathology than those that are present at exercise alone.