Insulin-like growth factor system (IGF system) has been reported to be associated with the variety of disorders of myocardial function. However, the effect of myocardial infarction (MI) on the IGF system has not been fully described. Thus, the present study was to investigate in more detail the changes of IGF system in the male rat following myocardial infarction (MI). Ligation of the left coronary artery was performed in male Sprague-Dawley male rats at 60 days of age. Control rats were obtained sham-operated animals. MI rats were sacrificed at 1, 3, 7, 14, and 30 day after ligation of left anterior descending coronary artery. Control rats were sacrificed on 30 day after thoracotomy. Myocardial infarct size was assessed by planimetry and perimetry. Serum and heart concentrations of IGF-I and -II were determined by radioimmunoassay. Serum levels of IGF-binding protein (IGFBP)-1 and IGFBP-3 were analyzed with a two-site immunoradiometric assay. Mean infarct size was 35.2~42.3% of the left ventricle after coronary occlusion in experimental groups. Serum levels of IGF-I were markedly reduced, but the levels of IGF-II were not altered in MI rats compared with shamligated controls. Serum IGFBP-I levels in MI rats were significantly increased at 1 and 3 day compared with sham rats. The levels of serum IGFBP-3 were significantly higher in the ligated rats. IGF-I levels of the infarct/periinfarct area of the left ventricle were significantly decreased in rats with myocardial infarction, whereas the levels of IGF-II remained unchanged. These results demonstrate that the IGF system is altered in the myocardial infarction and suggest that the IGF system plays an important role in the response of the heart to myocardial infarction.