This study investigated the effects of the oxidants hydrogen peroxide (H(2)O(2)) and 2,2'-dithiodipyridine (DTDP), and reductants, glutathione (GSH) and dithiothreitol (DTT), on the properties of the contractile apparatus of rat fast- and slow-twitch skeletal muscle fibres, in order to assess how oxidation affects muscle function. Skinned muscle fibres were activated in heavily-buffered Ca(2+) solutions. The force-[Ca(2+)] relationship before and after various treatments was fitted by a Hill curve described by the maximum Ca(2+)-activated force, pCa(50) (-log(10)[Ca(2+)] giving half-maximum force) and n(H) (the Hill coefficient). Exposing freshly skinned fibres to strong reducing conditions (i.e. 10 mM DTT or 5 mM GSH) had little if any effect on Ca(2+) sensitivity (pCa(50) or n(H)). The effect of oxidants H(2)O(2) and DTDP depended on whether the fibre was relaxed (in pCa > 9) or activated during the exposure. In both fast- and slow-twitch fibres a 5 min exposure to 10 mM H(2)O(2) at pCa > 9 had no effect on pCa(50), causing only a reduction in n(H). In contrast, when fast-twitch fibres were activated in the presence of 10 mM H(2)O(2) (or 100 microM DTDP) there was a substantial increase in pCa(50) (by approximately 0.06 and 0.1, respectively), as well as larger decreases in n(H) than occurred in relaxed fibres, with all effects being reversed by DTT (10 mM, 10 min). In slow-twitch soleus fibres, the activation-dependent effect of DTDP was even greater (pCa(50) increased by ~0.35), and it was found that the rate of reversal in DTT was also increased by activation. A separate important phenomenon was that fast-twitch fibres that had been oxidised with H(2)O(2) or DTDP (while either relaxed or activated) showed a paradoxical increase in Ca(2+) sensitivity (~0.04 and 0.25 increase in pCa(50), respectively) when briefly exposed to the endogenous reductant GSH (5 mM, 2 min). This effect was reversed by DTT or longer (> 20 min) exposure to GSH, did not occur in slow-twitch soleus fibres, and may contribute to post-tetanic potentiation in fast-twitch muscle. Maximum force was not affected by any of the above treatments, whereas exposure to a high concentration of DTDP (1 mM) did greatly reduce force production. These findings reveal a number of novel and probably important effects of oxidation on the contractile apparatus in skeletal muscle fibres.