Development of isometric tension in the preparations of glycerinated fibrils of different skeletal muscles in ATP + Ca2+ solution, as well as composition of muscle contractile and regulatory proteins were studied on white rats exposed on biosatellite "Kosmos-1129" for 18.5 days. Expression and trends of the changes in contractile properties of muscle preparations depend on their structural-functional specialization and biomechanical peculiarities. In each case adequate rearrangement of the composition of light myosin chains takes place. Changes in subunit composition of troponin-tropomyosin complex were recorded. High activity of catalase in muscle proteins disproportionate to the loss of their mass in single muscles was observed. The results obtained are in favour of possible adequate transformation of fibril phenotypes of some (antigravitational) muscles under the effect of space flight.