Oxygen-derived free radicals are known to take part in cardiac injury during post-ischemic reperfusion (I/R). Xanthine oxidase (XO) is closely associated with the generation of superoxide radicals. We have determined the distribution of XO in rat myocardium after ischemia (I) and I/R by immunocytochemical method using murine monoclonal antibody against XO (bovine milk) and by enzyme histochemistry (EHC) in situ. Frozen sections of periodate-lysine-paraformaldehyde (PLP) fixed myocardium after 15, 60 and 90 min ischemia and 15 min ischemia and 30 min reperfusion were processed for immunocytochemistry and EHC. In other experiments, rats were treated with allopurinol, an inhibitor of XO, and hearts were processed for immunocytochemistry. By immunoperoxidase and immunofluorescence methods, a deep staining of interstitial cells, capillary and small blood vessels was observed, but the staining intensity of these cells was increased after reperfusion, in comparison to the normal and ischemic heart tissue. In the electron microscope, an immunoperoxidase reaction product was seen in the cytoplasm of interstitial, endothelial and smooth muscle cells. Similarly, EHC studies by nitroblue tetrazolium staining showed an increase in enzymatic activity in the tissue after reperfusion. The allopurinol-treated I/R tissue exhibited reduced staining. The data suggest that XO activity increases during ischemia but intensifies after reperfusion. The enzyme is localized in interstitial cells, coronary vessel endothelium and smooth muscle cells. XO is constantly present in the interstitial cells of the myocardium and it is a new finding not previously reported. It is further suggested that myocardial interstitium may be one of the major sites where oxygen derived radicals are generated during ischemia.