The orthostatic hypotension after weightlessness has been suggested mainly because of the blood volume reduction, and decrease in stroke volume and cardiac output as its consequence. However, since bedrest is characterized as removal of orthostasis and exercise, which are major physiological stressors to the autonomic nervous system in normal daily life of humans, thus it may be possible that general autonomic activity itself would be deconditioned in the central nervous system after bedrest. Thus, we hypothesized that 20-day bedrest may modify the cardiovascular responses to cold pressor test, which has been used to evaluate the autonomic response and which might be independent of blood volume changes. And also it was hypothesized that if there were any changes in the response to the cold pressor test, they may be related to the changes of responses to lower body negative pressure (LBNP) test.