Studied were the bones of the limbs of 20 male rats of the Wistar line divided into 2 groups--one immobilized and one control. The latter was raised under the normal conditions of a vivarium, and the test group of 60-day-old rats was kept in individual cells for physiologic immobilization. The experiment lasted for a total of 390 day, i.e., at the time of killing and sampling the animals were at the age of 450 days. Each part of the limbs was measured severalfold at several sites by means of a slide-gauge and the data were processed statistically and via dispersion analyses. Besides, the structure of the bone tissue was investigated in histologic preparations, determining the density of the osteocytes. Measured was also the microhardness after Wikers of the bones of the two groups of animals, using a metallographic microhardmeter. It was established that all bones of the fore- and the hindlimb were shorter and in general smaller under the effect of hypokinesia. Immobilization was shown to affect chiefly the proximal bones of the limb skeleton. The growth of the bones decreased in hypokinesia to the detriment not so much of the cell elements than of the intercellular matter of the bone tissue. The microhardness of the bones of the immobilized rats was lower than that of the control ones.