BACKGROUND: Arrhythmias play an important role in reducing the performance of racing horse. There are no studies that maintained the same condition for all horses, and in previous studies, the conditions for all horses were not the same. OBJECTIVES: The purpose of this study was to evaluate the prevalence of arrhythmias during exercise include warm-up and trotting. METHODS: This study was carried out on 30 sport horses. Modified base-apex has been used for electrocardiogram recording. Electrocardiogram was taken by telemetry device in all conditions, all horses were examined in the electric lounge. Warm-up period was 10 minutes and consequently 10 minutes trotting was done, and ECG was recorded during exercise. RESULTS: 18 horses had SVPCs during warm-up and 9 showed SVPCs during trotting; also, 4 horses showed VPCs during trotting. AVB II happened in one horse during warm-up. CONCLUSIONS: The occurrence of SVPCs and VPCs during rest is abnormal, but it is common during physical activity. To better understand the importance of these arrhythmias more studies are needed.