The severity and duration of ischemia - reperfusion is hypothesized to play an important role in the ability of the heart to subsequently recover contractility. Permeabilized trabeculae were prepared from a rat model of ischemia - reperfusion to examine the impact on force generation. In comparison to the control perfused condition, the maximum force per cross-sectional area (Fmax) and the rate of tension redevelopment of Ca²⁺ -activated trabeculae fell by 71% and 44%, respectively, during ischemia despite the availability of a high concentration of ATP. The reduction in Fmax with ischemia was accompanied by a decline in fibre stiffness, implying a drop in the absolute number of attached crossbridges. However, the declines during ischemia were largely recovered following reperfusion, leading to the hypothesis that intrinsic, reversible post-translational modifications to proteins of the contractile filaments occur during ischemia - reperfusion. Examination of thin filament proteins from ischemic or ischemia - reperfused hearts did not reveal proteolysis of troponin I or T. However, actin was found to be glutathionylated with ischemia. Light scattering experiments demonstrated that glutathionylated G-actin did not polymerize as efficiently as native G-actin. Although tropomyosin accelerated the time course of native and glutathionylated G-actin polymerization, the polymerization of glutathionylated G-actin still lagged native G-actin at all concentrations of tropomyosin tested. Furthermore, cosedimentation experiments demonstrated that tropomyosin bound glutathionylated F-actin with significantly reduced cooperativity. Therefore, glutathionylated actin may be a novel contributor to the diverse set of post-translational modifications that define the function of the contractile filaments during ischemia - reperfusion.