Nitric oxide (NO) generated by skeletal muscle is believed to regulate force production but how this is achieved remains poorly understood. In the present work we tested the effects of NO-synthase (NOs) inhibitors on membrane current and intracellular calcium in isolated skeletal muscle fibres from mouse, under voltage-clamp conditions. Resting [Ca2+] and [Ca2+] transients evoked by large depolarizations exhibited similar properties in control fibres and in fibres loaded with tenth millimolar levels of the NOs inhibitor N-nitro-L arginine (L-NNA). Yet, the voltage dependence of calcium release was found to be shifted by ~15 mV towards negative values in the presence of L-NNA. This effect could be reproduced by the other NOs inhibitor S-methyl-L-Thiocitrulline (L-SMT). Separate experiments showed that the voltage dependence of charge movement and of the slow calcium current were unaffected by the presence of L-NNA ruling out an effect on the voltage sensor. A negative shift in the voltage dependence of calcium release with no concurrent alteration in the properties of charge movement was also observed in fibres exposed to the oxidant H2O2 (1 mM). Conversely the reducing agent dithiotreitol (10 mM) had no obvious effect on Ca2+ release. Overall, results indicate that physiological levels of NO exert a tonic inhibitory control on the activation of the calcium release channels. Changes in the voltage dependence of Ca2+ release activation may be a ubiquitous physiological consequence of redox-related modifications of the ryanodine receptor.