Morphologic studies were carried out on the skeletal musculature of two groups (test and control) of male rats of the Wistar line. The animals of the test group were kept when at the age of 60 days in individual cells for physiologic immobilization. Those of the control group were kept under normal conditions of raising. All animals were killed for sampling at the age of 450 days. Twenty-four hours after killing the following muscles were prepared and weighed for study: musculus longissimus dorsi, m. semitendinosus, m. semimembranosus, and m. triceps brachii. Micromorphologic and histometric investigations were conducted with the first three muscles, determining the quantitative ratio between the tissue components with the use of an 'integrational table' ISA. It was found that the skeletal muscles of rats kept for 390 days in the conditions of immobilization had a lower weight. The intramuscle fatty tissue of m. longissimus dorsi and m. semitendinosus as well as the connective tissue of m. longissimus dorsi showed higher weight, resp., amount in the immobilized animals, while the muscular tissue (the primary muscle bundles) was better developed in the control animals. The diameter of the muscle fibers of m. semitendinosus only was smaller in the immobilized rats. The variability in the thickness of the muscle fibers in the test group animals proved higher. Degenerative processes involved both the light and the dark fibers.