Modern methods of reparative surgery of the blood vessels enable the patency of the main arteries of the lower limbs to be restored comparatively quickly after acute occlusion. However, operative revascularization is not always effective, ischemic changes in the skeletal muscles are progressive, and under these conditions amputation of the affected limb cannot be avoided [3-6, 8, 15]. Development and implementation of timely ahd effective correction of acute ischemia of the limbs would undoubtedly be aided by knowledge of the dynamics: of the structural and metabolic changes taking place in skeletal muscles during acute occlusion of the main arteries.* Data in the literature on this problem do not deal with the problem as a whole, but only with certain of its aspects, and as a rule, they do not give an objective and quantitative evaluation of the morphological changes discovered [1, 7, 10, 12, 14]. The aim of this investigation was a combined quantitative morphological analysis of changes in the skeletal muscles during acute arterial occlusion of the limbs. EXPERIMENTAL METHOD Skeletal muscles from the leg of 36 dogs weighing 13-18 kg, with experimental acute occlusion of the terminal part of the aorta [2], were studied. The duration of ischemia was 0 (control), 3, 6, 9, and 12 h. Sections through the leg muscles, after embedding in paraffin wax, were stained with hematoxylin and eosin, by Van Gieson's Regaud's, and Lie's methods, and by the trichrome method of Goldner [9, 11]. Microscopic preparations were studied in ordinary and polarized light. Tissue stereologic analysis of volumes of intact and injured muscle fibers and the edematous stroma was carried out on sections stained by Regaud's method in 10 fields of vision on the "Struktura" apparatus (Central Design Bureau, Academy of Medical Sciences of the USSR). The following histochemical methods were used: the diaminobenzidine method for myoglobin [13], staining with Sudan Black B and Oil Red for phospholipids and neutral fats, the PAS reaction with amylase control to detect glycogen and neutral glycosaminoglyca ne. The enzyme-histologi cal investigation of the soleus muscle was undertaken on frozen section 10 # thick. Activity of the following enzymes was determined by the usual methods [11]: succinate (SDH), isocitrate (ICDH), malate (MDH), ~-giycerophospha te (~-GPDH), lactate (LDH), glucose-6-phosph ate (G-6-PDH), glutamate (GDH), and fi-hydroxybutyrate (fi-HBDH) dehydrogenases; NADand NADP-diaphorases , alkaline and acid phosphatases (AlP and AcP respectively), myosin ATPase, and phosphorylase. Changes in SDH, LDH, fl-HBDH, NAD, NADP, and ATPase were estimated quantitative on an MIF-7 integrating photometric microscope. In one transverse section 66 areas of red and white muscle fibers (R~F and WMF respectively) were measured photometrically. All numerical data were subjected to statistical analysis. The significance of differences between the values for the parameters was determined by Student's test. EXPERIMENTAL RESULTS After ischemia for 3 h damage to the muscle fibers was found. Tissue analysis showed changes in the ratio of the relative frequency of areas of intact and injured fibers and stroma compared with the control. In the control group, for instance, the ratio was 67:6:22%, and after ischemia for 3 h it was 6~.13:24%. The focal and more widespread increase in cross-striation of the muscle fibers will be noted (Fig. la), and in polarized light this was revealed as more intensive fluorescence of the A-disks. The time course of contraetura[ injuries to the muscle fibers could be discerned: localized areas of approximation of the A-disks, which merged *Academician of the Academy of Medical Sciences of the USSR.