Transforming growth factor 1 (TGF-1) alters myocardial gene expression resulting in myocyte hypertrophy, through activation of TGF--activated kinase (TAK1), a member of the mitogen-activated protein kinase kinase kinase (MAPKKK) family. We hypothesized that the TGF-1-TAK1-p38 MAPK pathway might be activated during ventricular remodeling after myocardial infarction (MI). One, 3, 7 and 14 days after ligation of the left anterior descending coronary artery, non-infarcted left ventricular tissue samples were obtained. Protein levels as well as mRNA levels of the signaling pathway, TGF-1, TGF--receptors and TAK1 increased in the non-infarcted myocardium in MI rats compared with sham-operated animals. Phosphorylation of MAPKK 3/6 (MKK3/6) and p38 MAPK, the downstream targets of TAK1, was also increased in the non-infarcted region. Moreover, an in vitro kinase assay revealed that the activated TAK1 in the non-infarcted myocardium was capable of activating recombinant MKK3/6, suggesting a causative role of TAK1 in the remodeling process. The activation of the TGF-1-TAK1-p38 MAPK pathway paralleled the transcriptional upregulation of cardiac markers for ventricular hypertrophy, -myosin heavy chain and atrial natriuretic peptide. TAK1 was mainly localized to cardiomyocytes, whereas TGF-1 receptors were observed in vascular smooth muscle cells and fibroblasts as well as cardiomyocytes. Thus, the TGF-1-TAK1-MKK3/6-p38 MAPK pathway in the cardiomyocytes of non-infarcted spared myocardium is activated following acute MI, and may play an important role in ventricular hypertrophy and post-MI remodeling in rats.