1. The effects of beraprost sodium (beraprost) on myocardial infarct size in an anesthetized open-chest canine model of regional myocardial ischemia and reperfusion were investigated. 2. Administration of beraprost (300 ng/kg/min, intravenously) to dogs 45 min after left circumflex coronary artery occlusion until 105 min after reperfusion resulted in a significant reduction in infarct size. 3. The values of control and beraprost treated infarct size expressed as a percentage of the total left ventricle were 15 +/- 3% and 4 +/- 2%, respectively. 4. Reperfusion arrhythmia, plasma creatine phosphokinase (CK) and lactate dehydrogenase (LDH) level were significantly suppressed by treatment with beraprost. 5. By histological examination, beraprost proved to reduce neutrophil migration in the ischemic myocardium after 5 h reperfusion. 6. Therefore, it is suggested that the cytoprotective effect of beraprost during myocardial ischemia and reperfusion may be the consequence of the inhibition of neutrophil migration.