Although clinical diabetes mellitus is obviously a high risk factor for myocardial infarction, there is disagreement about the sensitivity of ischemic injury of an infarcted myocardium in experimental studies. The present study evaluated the influences of different durations of hyperglycemia on ischemic and reperfusion injuries of the myocardium, and focused on extracellular signal-regulated kinase 1/2 (ERK1/2), which plays an important role in the intracellular signaling pathway and is reported to be associated with myocardial protection against heart injury. Short- and long-term hyperglycemias were induced in rats by streptozotocin (STZ) injection and the rats were examined 4 (4WDM) and 20 weeks (20WDM) after the treatment. Ischemia and reperfusion were induced by occlusion and reperfusion (I/R) of the left coronary artery (LCA). I/R-induced infarct size was determined using triphenyltetrazolium chloride (TTC) staining. After 20 weeks of STZ treatment (20WDM+I/R), the infarct size in the rat heart increased by 65.2 +/- 4.3%, whereas after 4 weeks of STZ treatment (4WDM+I/R), the infarct size decreased compared with the time-matched I/R group (43.1 +/- 3.6% and 59.5 +/- 5.6%, respectively). The number of dead myocytes including necrotic and apoptotic cells was determined using horseradish peroxidase (HRP) and terminal deoxynucleotide nick-end labeling (TUNEL) methods. The number of dead myocytes decreased in the 4WDM+I/R group, while the number of dead myocytes increased markedly in the 20WDM+I/R group, compared with the time-matched I/R group. The increment of ERK1/2 phosphorylation in the 4WDM group and the slight enhancement of this phosphorylation by I/R treatment were observed by western blotting. However, in the 20WDM group, the level of ERK1/2 phosphorylation reduced by approximately 1/3 compared with the time-matched control group; moreover, I/R treatment did not enhance the phosphorylation level. This study demonstrated that short- and long-term hyperglycemias exert opposite influences on ischemic myocardial injury, and these contradictory influences may depend on an ERK1/2-mediated intracellular signaling pathway.