S-Nitrosoglutathione (GSNO) is generated in muscle and may S-glutathionylate and/or S-nitrosylate various proteins involved in excitation–contraction (EC) coupling, such as Na+-K+-ATPases, voltage-sensors (VSs) and Ca2+ release channels (ryanodine receptors,RyRs), possibly changing their properties. Using mechanically skinned fibres from rat extensor digitorum longus muscle, we sought to identify which EC coupling processes are most susceptible to GSNO-modulated changes and whether these changes could be important in muscle function and fatigue. For comparison, we examined the effect of other oxidation, nitrosylation, or glutathionylation treatments (S-nitroso-N-acetyl-penicillamine (SNAP), hydrogen peroxide,2,2-dithiodipyridine and reduced glutathione) on twitch and tetanic force, action potential (AP) repriming, sarcoplasmic reticulum (SR) Ca2+ loading and leakage, and contractile apparatus properties. None of the treatments detectably altered AP repriming, indicating that t-system excitability was relatively insensitive to such oxidative modification. Importantly, the overall effect on twitch and tetanic force of a given treatment was determined primarily by its action on Ca2+ sensitivity of the contractile apparatus. For example, S-nitrosylation with the NO• donor,SNAP, caused matching decreases in the contractile Ca2+ sensitivity and twitch response, and GSNO applied ∼10 min after preparation had very similar effects. The only exception was when GSNO was applied immediately after preparation, which resulted in irreversible decreases in twitch and tetanic responses even though it concomitantly increased Ca2+ sensitivity by∼0.1 pCaunits, the latter evidently due to S-glutathionylation of the contractile apparatus. This decrease in AP-mediated force responses was due to impaired VS–RyR coupling and was accompanied by increased Ca2+ leakage through RyRs. Such oxidation-related impairment of coupling could be responsible for prolonged low frequency fatigue in certain circumstances.