Active oxygen species including hydrogen peroxide (H2O2) play a major role in ischemia-reperfusion injury. In the present study, changes in myocardial H2O2 content as well as its subcellular distribution were examined in rat hearts subjected to ischemia-reperfusion. Isolated perfused rat hearts were made globally ischemic for 20 or 30 minutes and were reperfused for different durations. H2O2 content in these hearts was studied biochemically and changes were correlated with the recovery of function. These hearts were also analyzed for subcellular distribution of H2O2. Optimal conditions of tissue processing as well as incubation medium were established for reacting cerium chloride with H2O2 to form cerium perhydroxide, an insoluble electron-dense product. The chemical composition of these deposits was confirmed by x-ray micro-analysis. Global ischemia caused complete contractile failure in minutes and after 30 minutes of ischemia, these was a > 250% increase in the myocardial H2O2 content. Depressed contractile function recovery in the early phase of reperfusion was accompanied by approximately a 600% increase in the myocardial H2O2 content. Brief pre-fixation with low concentrations of glutaraldehyde, inhibition of alkaline phosphatase, glutathione peroxidase, and catalase, post-fixation but no post-osmication, and no counterstaining yielded the best cytochemical definition of H2O2. In normal hearts, extremely small amounts of cerium hydroperoxide precipitates were located on the endothelial cells. X-ray microanalysis confirmed the presence of cerium in the reaction product. Ischemia resulted in a stronger reaction, particularly on the sarcolemma as well as abluminal side of the endothelial cells; and upon reperfusion, cerium precipitate reaction at these sites was more intense. In the reperfused hearts, the reaction product also appeared within mitochondria between the cristae as well as on the myofibrils, but Z-lines were devoid of any precipitate. The data support a significant increase in myocardial H2O2 during both the phase of ischemia and the first few minutes of reperfusion. A stronger reaction on the sarcolemma and abluminal side of endothelial cells may also indicate enhanced H2O2 accumulation as well as vulnerability of these sites to oxidative stress injury.