The results of study of the crew members of the spaceships Soyuz are described, and the effects of weightlessness on reflex excitability, muscular tone and muscle contractibility discussed. A certain decrease in postural muscular tone and strength, increase in reflex excitability at rest and increase in bioelectric activity of muscles at work has been found in the cosmonauts after their stay in a weightless environment. The circumference of the lower extremities decreased.