Heart rate variability (HRV) is a noninvasive tool to study cardiac autonomic regulation. In recent years there is growing evidence that training affects HRV indices (1). The active orthostatic task has been used to evaluate HRV in trained and untrained subjects, but the results are contradictory. Studies using second change of body position (lying down after been in upright posture) are scarce. Aim of this investigation was to assess the differences in HRV between endurance trained athletes and sedentary controls at rest and during active orthoclinostatic test. Sixteen endurance trained athletes (age=22.0±2.42 years) and sixteen (age=21.5±1.03 years) untrained controls participated in the study. In the supine position after 20 min of rest RR intervals of each subject were recorded for 5 min with Polar S810i heart rate monitor. After that subjects actively stood up and remained in upright position for 8 min. The subjects then returned in the supine position for 8 min (orthoclinostatic test). RR intervals were collected for 5 min from 3th to 8th minutes during upright position and second supine position. The power spectrum indices included: LF (low frequency, 0.04 Hz-0.15 Hz) and HF (high frequency, 0.15-0.40 Hz), both expressed in absolute (ms2) and in normalized units (nu). LF/HF ratio was calculated. Mean HR (bpm) and Mean RR (ms) were also determined. VO2max in athletes was significantly higher in athletes than in untrained. During supine rest Mean HR was lower (p<0.001) in athletes than in untrained (58.3±6.29 vs. 68.78±8.66 bpm). Mean RR was higher (p<0.001) in athletes (1046.47±113.42 vs. 890.17±116.98 ms). HF ms2 was higher (p<0.05) in athletes than in untrained (2063.22±1598.77 vs.1102.60±849.04). HF nu was higher (p<0.05) in athletes than in untrained (57.86±16.75 vs. 43.45±6.77). LF nu was lower (p<0.05) in athletes than in untrained (42.13±16.75 vs. 56.55±6.77). LF/HF ratio was lower (p<0.05) in athletes than in untrained (0.88±0.57 vs. 1.37±0.47). There was no difference between the two groups in regard to spectral HRV indices after the two changes in body position. The differences in HRV indices between the two groups at rest (first supine position) suggest more pronounced parasympathetic influence on cardiac regulation in athletes in comparison with untrained. There is tendency showing less marked sympathetic cardiac influences in athletes. Active standing eliminates the differences in HRV between trained and untrained subjects. The lack of differences between the groups remains after assuming second supine position which may be caused by the saturation of HRV (2). These results suggest that the more complex autonomic heart regulation elicited by two changes in body position suppress the differences in HRV related to endurance training. This effect during orthoclinostatic test requires further study.