The first studies of space flown rats revealed that in m.soleus (SOL) slow-twitch fibers (ST) were reduced in size more pronouncedly than fast-twitch fibers (FT). The same differences in the fiber atrophy levels were found in rats after hind-limb suspension. At the same time, the studies on humans exposed to real and simulated weightlessness demonstrated that at least in m.vastus lateral (VL) the ST and FT fibers' levels of atrophy were almost similar. The discrepancies in the results of human and rat studies are believed to be explained by: i/species-specificity in muscle plasticity pattern; ii/differences in characteristics of muscles usually being under rat and human studies (SOL vs VL); iii/observation of different stages of the atrophy development in different species; The study was aimed to test these hypotheses.