The article presents normal values of the major parameters of lower limb veins in cosmonauts during preparations for space flight and volunteers leading a common life. The authors report the results and discuss the causes for differences in normal leg venous parameters in these groups of subjects. Incomparability of measurements made in cosmonauts and common people is demonstrated. Changes in lower limb veins of a cosmonaut in microgravity can be evaluated only relative to his/her normal values of the major venous parameters (capacitance, compliance and filling) before flight.