The study explored the hypothesis that mares, which are more sensitive than stallions, react to stress with higher increase in heart rate (HR).A group was studied of 101 clinically healthy Standardbred trotters aged 11-18 months, during their daily training routine. The horses were divided into two groups according to sex (51 colts and 50 fillies). All the horses took part in a standard training session consisting of harnessing to the sulky, moving from the stable and a 45 minutes exercise on a sand track. Before the start of training session a belt with a transmitter for telemetric HR registration was placed on each horse. The measured HR did not show any differences in HR between colts and fillies either at rest, during handling and harnessing as well as during exercise and after it.