During parabolic flight short periods of microgravity and hypergravity are created. During standing position time domain analysis showed higher vagal modulation of the autonomic nervous system in microgravity compared to hypergravity. We hypothesised that this behaviour could better be unravelled by analysis of frequency domain heart rate variability (HRV) techniques. During parabolic flights a subject is exposed to 20-25 sec periods of microgravity (at the top of the parabola), preceded and followed by 20 sec duration episodes of hypergravity. No significant differences were found in the frequency parameters in supine position for each of the phases. In standing position higher values for Total Power, high frequency (HF), low frequency (LF), LF/HF and HF% in 0 g phase were found in comparison with the 3 other phases. LF% was significant lower in standing position at 0 g compared with 1.8 g. These results show that despite the time window limitations, frequency analysis is still possible on ultra short data segments.