To evaluate if changes in athletes' physical fitness due to seasonal training are associated with changes in cardiovascular autonomic control, nine swimmers (three males and six females; aged 14-18 years) were evaluated before and after 5 months of training and competitions. Maximal oxygen consumption (VO2max) and ventilatory threshold were determined during a maximal test; heart rate (HR) and blood pressure (BP) variabilities' power spectra were calculated at rest (supine and sitting positions) and in the recovery of two exercises at 25 and 80% pre-training VO2max. At the end of the season: (a) VO2max and ventilatory threshold increased respectively by 12 and 34% (P<0.05); (b) at rest, HR decreased by 9 b min(-1) in both body positions, whereas BP decreased in supine position only by 17%. No change in low frequency (LF, 0.04-0.15 Hz) and high frequency (HF, 0.15-1.5 Hz) normalized powers and in LF/HF ratio of HR variability and in LF power of systolic BP variability was observed. In contrast, a significant increase in HF alpha-index (about 12 ms mmHg(-1)) was found; (c) during recovery no change in any parameter was observed. Seasonal training improved exercise capacity and decreased resting cardiovascular parameters, but did not modify vagal and sympathetic spectral markers. The increase in alpha-index observed at rest after the season and expression of augmented baroreflex sensibility indicated however that HR vagal control could have been enhanced by seasonal training. These findings suggested that autonomic system might have played a role in short-term adaptation to training.