We studied the influence of training on autonomic nervous function in the horse. For this purpose, Holter electrocardiogram (ECG) was recorded before and after training from 24 Thoroughbred horses (2-year-olds) and autonomic nervous function was evaluated by the power spectral analysis of heart rate (HR) variability. We obtained HR, low-frequency (LF) power, high-frequency (HF) power, and LF/HF ratio from recording. We set LF at 0.01–0.07 Hz and HF at 0.07–0.6 Hz. The HF power is thought to reflect primarily parasympathetic nervous function. Both the sympathetic and parasympathetic nervous systems have been shown to contribute to the LF power. The LF/HF ratio is considered as an index of the cardiac sympathovagal balance. Second degree atrioventricular blocks were found in the ECG of 3 and 5 horses before and after the training period, respectively. Ventricular premature depolarisations were detected in a horse after the training period. Heart rate decreased at night The lowest HR values had a tendency to appear in the early morning. Both the LF and HF power tended to be higher at night However, the LF/HF ratio was almost the same throughout the day. The HR was significantly decreased by training. Although the LF power and LF/HF ratio were significantly increased, the HF power was not changed by training. These results suggest that parasympathetic nervous activity may be fully activated even before training in this species.