Changes in the spectra of heart rate and systolic blood pressure variabilities (HRV and SBPV) were recorded during head-down bed rest and head-up tilt (HUT) in 15 healthy young men. After bed rest, five subjects failed to complete HUT test. During bed rest, the low-frequency (LF) and high-frequency (HF) powers of HRV and SBPV spectra were all significantly decreased, the ratio of low-frequency to high-frequency power of HRV spectrum (LF:HFHRV) increased on the 16th day. During the first 6min of HUT after bed rest, the heart rate was faster, but the LF:HFHRV and the LF power of SBPV spectrum in all subjects were similar to the data during HUT before bed rest. The diastolic and mean arterial pressure were significantly increased during HUT except in those who failed to complete the HUT test after bed rest. The results suggest that the peripheral vascular sympathetic activity and the cardiac parasympathetic activity are decreased with a relative increase in the cardiac sympathetic activity during bed rest, but the cardiovascular autonomic responsiveness during HUT after bed rest is basically preserved.