Analysis of ultrastructural changes in diaphragm muscle and synaptic apparatus after 7- and 14-day biosatellite spaceflight showed destructive and atrophic changes that developed starting from in the early terms of exposure. Structural alterations of different severity were revealed in all elements of the muscle (muscle fibers, neuromuscular junctions, intramuscular nerves, and blood vessels) and were paralleled by activation of regeneration processes.