Little is known about changes in the amount of iron in the intracellular low molecular weight pool, which catalyzes the Fenton reactions during reperfusion after ischemia. In this study a new approach is presented to measure low molecular weight iron and it is applied to normal hearts during ischemia and to iron-loaded hearts during anoxia and reoxygenation. The results of this study show that (a) during ischemia in normal hearts a progressive 30-fold increase occurs in low molecular weight iron after 45 min of ischemia, whereas (b) during 45 min of anoxic perfusion the low molecular weight iron does not increase. This means that the reductive release from the storage protein ferritin is greatly enhanced by the acidification that occurs during ischemia. (c) Anoxic perfusion of iron-loaded hearts does increase low molecular weight iron and there is a further increase upon reoxygenation, which is prevented by (+)-cyanidanol-3. Based on these findings it is concluded that oxygen deprivation enhances the susceptibility of rat hearts to oxygen radicals by increasing the amount of catalytic, ferrous iron in the low molecular weight pool.