The aim of the present work is to elucidate the effect of aerobiac training on cardic autonomic function and to clarify whether there is any association between the changes in cardiac regulation and the heart rate dynamics and orthostatic tolerance during LBNP testing. To achieve this, the heart rate variability (HRV) signals obtained from a group of eight students before and after a 6-mon aerobic training, as well as from six athletes (medium- and long distance runners) were analyzed by conventional spectral, dynamic spectral and non-linear analysis. Our results showed that the conventional AR spectral analysis could not provide data with significance, owing to its greater variance and inherent limitation in being able to reflect only the average statistical characters over a certain period. While from the data obtained by use of the time-varying AR spectral analysis we could follow the time course of cardiac vagal withdrawl and sympathetic excitation during LBNP exposure. Regarding the non linear methods used, beta estimates didn\'t provide any significant result, but the ApEn analysis of the HRV signal could detect subtle changes in heart rate dynamics associated with aerobic training. Moreover, after aerobic training, the increments delta ApEn and delta DNP during LB NP testing were closely correlated. Our results would have important implications for further work in elucidating the effect of aerobic training on heart rate dynamics and improving the work on HRV signal analysis.