Oxidants have been suggested to enhance contractile function in unfatigued muscle. In this study we aimed to determine the effect of oxidants on 'chemically skinned' diaphragm muscle fibre bundles. The sarcoplasmic reticulum and contractile proteins were exposed to superoxide anions (O2-) and hydrogen peroxide (H2O2) under controlled conditions. Application of O2- initially increased maximum Ca2+-activated force but subsequently reduced maximum Ca2+-activated force without altering myofilament Ca2+ sensitivity. Unlike myocardium, caffeine-induced Ca2+ release from the sarcoplasmic reticulum was also inhibited by O2- exposure in diaphragm fibre bundles. Application of H2O2 also increased maximum Ca2+-activated force but had additional effects on resting tension (which increased to 25 % of the control maximum Ca2+-activated force). H2O2 was without effect on myofilament Ca2+ sensitivity or caffeine-induced Ca2+ release from the sarcoplasmic reticulum. These data demonstrate that oxidants can potentiate contractile force in the diaphragm through a direct action on the contractile proteins. The potentiation of force is not sustained, however, and under these conditions the detrimental effects of O2- on Ca2+ release from the sarcoplasmic reticulum combined with the effects of oxidants on the contractile proteins will ultimately compromise excitation-contraction coupling in the diaphragm. Experimental Physiology (2001) 86.2, 161-168.