It was shown that eccentric contraction is associated with skeletal muscle damage including cytoskeletal proteins degradation. Moreover, muscle pain and weakness are present that is probably the reason of decrement in muscle performance. We suggested that NO level decreases during eccentric contraction which leads to cytoskeletal proteins degradation in skeletal muscle. Our study was aimed to determine if NO level increase during muscle eccentric contraction can prevent proteolysis and improve work capacity after the contraction. Male Wistar rats were divided into 5 groups: control group (C group, 260-302g, n=8), control group treated with L-arginine, NO precursor (500 mg/kg body wt. per os for 2 days; CA group, 270-305g, n=8), downhill motor-driven treadmill running group (40 min of treadmill running downhill at 16°, 20 m/min; R group, 280-310g, n=10), groups of treadmill running rats with L-arginine (500 mg/kg body wt. per os for 2 days before the running; RA group, 270-298g, n=10) or L-NAME, nNOS inhibitor, administration (90 mg/kg body wt. per os for 2 days before the running; RN group, 265-289g, n=10). The experiment was conducted according to the rules of biomedical ethics (protocol No. 264 of March 5, 2009 was approved by the Russian Academy of Sciences Committee on Bioethics). The rats were sacrificed by nembutal overdose (75 mg/kg body wt.) next day after the running, the m. soleus was immediately frozen in liquid nitrogen. EPR spectroscopy revealed that NO level in CA group was significantly greater than that in C group (p<0,05). When tested work capacity it was considerably greater in C and RA groups as compared to R and RN groups (p<0,05). Contents of cytoskeletal proteins dystrophin and desmin were tested by immunohistochemistry and western blotting respectively. Degradation of the proteins was significantly greater in both R and RN groups. L-arginine administration in RA group prevented the proteolysis after eccentric contraction. We determined by means of real-time RT-PCR that contents of μ-calpains mRNA in R and RN were significantly greater in comparison with C group. L-arginine administration in RA group considerably decreased μ-calpains mRNA expression as compared to R group (p<0,05). Having analysed E3-ligases, MAFbx and MuRF-1, we showed that its mRNA expressions in RA group didn’t differ from that in control group, but it was significantly lower in R and RN groups than that in C and RA groups (p<0,05). nNOS mRNA expression was greatly higher in R and RN groups as compared to C group (p<0,05). It is concluded that NO precursor administration in time of eccentric contraction increases work capacity and prevents proteolysis of cytoskeletal proteins, dystrophin and desmin. However nNOS inhibition at the eccentric contraction lead to dystrophin and desmin degradation and the capacity decrease. The work was supported by grant RFBR 08-04-01599a