BACKGROUND/HYPOTHESIS: During parabolic flight, in the standing position, changes are partly due to an acute shift in fluid between the lower extremities, the head and the thorax (Vaida P, et al. J Appl Physiol 1997; 82:1091-7; and Bailliart O, et al. J Appl Physiol 1998; 85:2100-5). We hypothesized that modifications of parasympathetic activity associated with changes in hydrostatic pressure gradients induced by changes in gravity could be detected by analysis of short time periods. METHODS: We assessed heart rate variability (HRV) in 11 healthy volunteers by indices of temporal analysis (NN, SDNN, RMSSD) and normalized indices such as coefficients of variation CV-SDNN and CV-RMSSD and ratio SDNN/RMSSD. A lower body negative pressure (LBNP) at -50 mm Hg was randomly applied during the microgravity phase (0 Gz) to counteract the lack of hydrostatic pressure in the lower part of the body. RESULTS: NN, CV-SDNN and CV-RMSSD decreased during hypergravity phases and increased during microgravity and during early normogravity (1 Gz) period at the end of parabolas. With LBNP changes are less pronounced at 0 Gz and in the 1 Gz post parabolic period. CONCLUSION: We concluded that parasympathetic nervous activity is recordable by temporal analysis of HRV during short periods of time. LBNP applied during 0 Gz phase reduced the parasympathetic activation at 0 Gz and post parabolic 1 Gz.