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We investigated whether modifications of sympathetic and parasympathetic activity during parabolic flight could be detected using a time-variant model, able to estimate low (LF, 0.04-0.14 Hz) and high (HF, 0.14-0.35 Hz) frequency spectral components on a beat-to-beat basis. 10 subjects were studied with and without lower body negative pressure (LBNP, -50 mmHg). ECG signal and Gz load were digitized at 500 Hz and RR interval variability series were generated. Beat-to-beat estimates of mean RR, variance, absolute LF and HF power were obtained. One-way ANOVA was used to compare the values obtained during the different phases of parabolic flight sequence (I: initial 1 Gz, II: Initial 1.8 Gz, III: 0 Gz, IV: final 1.8 Gz, V: final 1 Gz), with p<0.01 assumed as significant. Without LBNP, total and LF power increased significantly during 0 Gz to 1.69±1.41 and 2.87±4.66 respectively (mean±SD, normalized by phase I value). With LBNP, the change in total and LF power during 0 Gz (1.38±1.37 and 1.54±1.04 respectively) reached significance only with phase II and phase V. Phase I HF power was higher than in the other phases, both without and with LBNP. During parabolic flight, parasympathetic activity was reduced with changes in Gz. LBNP reduced the sympathetic activation at 0 Gz.