Ischemia/reperfusion (I/R) and preconditioning of the heart by coronary artery occlusions increase expression of heat shock protein 70 (HSP 70). Because free radicals are generated during I/R, we hypothesized that the oxidant stress might contribute to an increased expression of HSP 70. Isolated rat hearts were perfused with free radical-generating systems such as xanthine/xanthine oxidase (X/XO), irradiated rose bengal (RB) generating singlet oxygen, and H2O2 for 15 min followed by 30 min of recovery period. Significant decrease in developed pressure and coronary flow occurred after perfusion with X/XO, H2O2, and RB. During I/R, the developed pressure and coronary flow were 60 +/- 8 and 80 +/- 5%, respectively, of control, which improved significantly with superoxide dismutase. The expression of HSP 70 mRNA increased over 13-fold in hearts perfused with X/XO, 6- to 7-fold with RB, and over 5-fold with H2O2. With I/R, an over 10-fold increase in HSP 70 mRNA was observed, which decreased significantly in the presence of superoxide dismutase. These results demonstrate that oxidant stress directly increases HSP 70 mRNA in the rat heart. It is concluded that one of the potential mechanisms of expression of HSP 70 by I/R may be oxygen radicals.