In this study we investigated the influence of body mass on the daily rhythm of total motor activity in ponies and horses. Six clinically healthy Shetland pony breed with a body mass of 180 ± 30 kg and six healthy horses Sella Italiana breed with a body mass of 530 ± 20 kg were used. Animals were equipped with actigraphy-based data loggers, Actiwatch-Mini® to record total activity. The application of two-way ANOVA showed a highly significant effect of time of day and body mass on motor activity. Cosinor analysis identified the periodic parameters and their acrophases during the two days of monitoring. Mesor and amplitude were statistically higher in horses than in ponies and no statistically significant differences was found about acrophases and robustness. In conclusion we suppose that the difference in the amount of total motor activity, in animals that differ in breed, as pony and horse, may lie in physical differences.