The study (consisting of two stages: I – when horses were entered into Training Centre, II – before final performance trait) included 253 half-bred stallions that were trained and given into performance traits in Biały Bór and Bogusławice Training Centres. The quality of mental traits in all horses was assessed using optic test by Budzyński’s method and simultanously measuring their heart rate (in rest and in test) by telemetric method. A statistically significant influence of the studied stallions’ sire origin on the results in optic test and the level of the heart rate measured at particular stages of study was found. The stay at the Training Centre had a good effect on the quality of nervous reactions and behaviour shown by the assessed stallions.