Metabolic inhibition in isolated perfused cardiomyocytes leads to cessation of contractile activity followed by rigor contraction. A fall in ATP is widely accepted to be the underlying cause for these changes. However, it is likely that calcium loading and generation of reactive oxygen species (ROS), known to occur during metabolic inhibition, would also trigger rigor contracture. Interestingly, all of these cellular changes are known to induce the opening of mitochondrial permeability transition pore (MPTP). The aim of this work was to determine whether inhibiting MPTP with cyclosporin A (CsA) during metabolic inhibition influences the time to stop beating and to go into rigor. Cardiomyocytes were isolated from rat hearts using enzymatic dispersion with collagenase and protease. Cells were perfused under a microscope with HEPES buffer at 34°C, stimulated at 0.2Hz and viewed on a monitor. Metabolic inhibition was simulated using 2.5mM NaCN in glucose free HEPES buffer, pH 7.4 with and without CsA (0.2µM). Data is expressed as mean ± standard error. CsA did not alter contractility or death rates of myocytes perfused with normal buffer. Exposure of adult cardiomyocytes to metabolic inhibition induced the characteristic arrest of contractility followed by rigor (a 30% decrease in length). The presence of 0.2µM CsA increased the time to stop beating (25.5±1.3 vs. 35.4±1.7 min) and going into rigor (28.8±1.3 vs. 44.2±2.7 min). When metabolic inhibition was limited to 20min followed by 20 min reperfusion with buffer, the presence of CsA during this period significantly increased the time to go into rigor during reperfusion (from 37.6±1.5 to 47.1±0.7 min). This preliminary data suggest that MPTP could have a role in triggering rigor contracture during metabolic inhibition.