The aim of this study was to analyze the correlation between the results of a horse's behavior assessment based on the Brzeski’s classification and heart rate at so-called threshold moments of natural training. The study included 120 horses of three breeds: Thoroughbred horses (n = 40), Purebred Arabian horses (n = 40), and Angloarab horses (n = 40). Each group included the same number of mares and stallions. The experiment involved natural training aimed at moun- ting a horse, and it lasted for a maximum of three consecutive days. Threshold moments were identified: first following after the trainer, first moving around the trainer, saddling, girth tighte-ning, mounting and adopting a seated position by the trainer. Heart rate (HR) was registered using Polar S810 devices. Equine behaviour was scored from 1 to 5 according to a modified Brzeski’s classification (1 – dangerous aggressive attitude, 5 – excellent cooperation with a trainer). Mul-tivariate analysis of variance (ANOVA, GLM) was conducted. Significant differences between the means were determined using the Tukey’s test. It was found that the heart rate at threshold mo-ments of the training and the behaviour score given by the trainers emphasized the racial distinc-tion of the Angloarab horses, in which the implementation of the natural training methods may be problematic. Modified Brzeski’s classification, usedas a method of evaluating the behaviour of young horses, may complement the physiological parameter analysis. Particular attention should be drawn to the assessment of the equine behavior during the first saddling. The importance of this assessment is confirmed by its significant correlation with the heart rate.