Hydration status of humans was assessed by means of bioimpedancemetry on board the space station or under the conditions of antiorthostatic hypokinesia (AOSH). Water compartments of the body were decreased in a cosmonaut at the seventh day of a ten-day-long flight to the same degree as in a group of six testers by the seventh day of AOSH (−8°): the amount of total body fluids and intracellular and extracellular volumes were decreased by 5.6–6.5% as compared to the baseline level. The changes in body composition of a cosmonaut during flight were similar to the changes observed in testers during AOSH: lean body weight, which was determined by bioimpedancemetry, was insignificantly decreased, whereas the adipose component of body weight was, on the contrary, increased. It was concluded that the hydration level of the human body was decreased and the amount of body fat was increased during a short-term space flight. It was also shown that the hydration status and composition of the human body were changed in a similar way under the conditions of both AOSH and space flight, which indicates that this ground-based model is adequate for simulation of hydration changes caused by microgravity.