The purpose of this study was to investigate how gravity level affects the excitability of the soleus muscle (SOL) motoneuron pool to Ia afferent input while erect posture is maintained in humans. Three healthy male subjects participated in an experiment whereby three different gravity conditions [microgravity (MG), normal gravity (NG), and hypergravity (HG)] were imposed using a parabolic flight procedure. The SOL H-reflex was evoked every 2 s while the subjects kept an erect posture. The stimulus intensity was controlled automatically on a real-time basis by personal computer to induce the constant amplitude of M-wave (10+/-5% of maximal M-wave amplitude). The background electromyographic activity (BGA) of the SOL was largest during HG, while it was almost absent during MG. The SOL H-reflex amplitude was significantly larger during HG and MG than during NG ( P<0.05). During NG and HG, there was a linear relationship between the BGA and the H-reflex amplitude; the difference in the SOL H-reflex amplitude between both gravity conditions could be explained in terms of the BGA level. However, during MG, despite the absence of BGA, the SOL H-reflex amplitude was larger than that during NG. Furthermore, when the subjects voluntarily activated the SOL by applying a load to the lower limb joints and spine by pulling a handle upward, this H-reflex enhancement almost disappeared. These results suggest that the somatosensory systems detecting a load at the lower limbs and/or vertebral column might play a role in reducing the excitability of the SOL motoneuron pool to Ia afferent inputs by presynaptic inhibition.