To clarify the relationship between postural instability and silent period (SP), we studied the variation of SP from soleus (SOL) and gastrocnemius (GAS) muscles on various standings. Subjects were eight healthy males, with a mean age of 23.5 +/- 2.2 (21-27) years. Standings in this study was regulated with ten kinds of situations provided by visual information, supporting or not by a finger and a width of base of support. SP evoked by single stimulation to tibial nerve at the popliteal fossa was recorded from SOL and GAS of dominant side during ten kinds of standings in random. The raw data were averaged 30 times. SP was calculated the duration from the artifact due to electrical stimulation to re-starting the electromyographical bursting of tonic muscle contraction under 100 or 200 microV|div on a screen. As a result of this study, there were not any statistical significant changes in SP from both SOL and GAS (one-way ANOVA: F = 1.797, F = 1.786) among ten kinds of standing. It is thought that a variation of SP may reflect the magnitude of facilitation or disinhibition of the central nervous system including spinal, brainstem or motor cortex. As the result of this study in healthy persons, it was suggested that the degree of facilitation or disinhibition of central nervous system related to SP from SOL and GAS was not different on ten kinds of standings regulated by visual information and a width of base of support.