Electrocardiography (ECG) is critically important for monitoring of heart rhythm and for diagnosis of arrhythmias. Holtermonitoring is particularly valuable when anamnestic dataindicate arrhythmia, syncope or when arrhythmia cannot bediagnosed using ECG assessment before, during or after training. The aim of this study was to use Holter monitoring over24-hour intervals to investigate ECG parameters (amplitudesand intervals) in sport and work group horses at rest and afterphysical activity. Seventeen Arabian horses and 20 BosnianMountain ponies comprised sport and work horse groups forthe study, respectively. All horses were outfitted with HolterECG Televet 100 devices for continuous electrocardiogram recording. Exercise for sport group horses involved enduranceriding on a 10 km long cross-country trail. Work group horsesextracted of logs (approx. 250 kg each) from limited accesslogging areas on a trail up to a few hundred meters long. Statistical analysis of mean ECG parameters between sport andworkhorses before and after physical activity was performedby independent Student’s t-test (two-tail, assuming unequalvariances). p<0.05 was used as the level for significance. Results indicate that before and after physical activity, T amplitude was higher in sport compared to work group horses.Further, after physical activity, the QRS complex interval waslonger in sport compared to work group horses.