The accumulation of polymorphonuclear leukocytes (PMN) into a canine occlusion/reperfusion model of myocardial infarction (MI) has been quantitated using a spectrophotometric assay for the PMN-specific enzyme myeloperoxidase (MPO). In anaesthetized beagle dogs subjected to 1 h of occlusion of the left anterior descending coronary artery followed by coronary reperfusion for 1 h, MPO activity was found in fourfold greater amounts in infarcted myocardium as opposed to normal myocardium. MPO activity in infarcted myocardium increased with coronary reperfusion in a time-related manner, the greatest accumulation occurring at 5 h of coronary reperfusion. The time-related increase in MPO activity in ischemic myocardium was closely correlated with the observed accumulation of PMN as assessed by histological sectioning, thus supporting the specificity of the method. Pretreatment of dogs with ibuprofen (12.5 mg/kg i.v.) did not alter the MPO content and therefore PMN invasion of infarcted myocardium, a finding confirmed by light microscopy. However, pretreatment with ibuprofen increased myocardial infarct size (from 11.5 to 37.4%, p less than 0.01) and increased the incidence of haemorrhagic infarction and ventricular fibrillation. In conclusion, our studies demonstrate that MPO can be used as a sensitive and quantitative marker of PMN accumulation into an evolving MI. Furthermore, we have demonstrated that PMNs rapidly infiltrated injured myocardium in a time dependent manner and this accumulation was unaffected by ibuprofen pretreatment.