Connective tissue growth factor (CTGF), a 36- to 38-kDa peptide, is selectively induced by transforming growth factor-beta and has been suggested to contribute to tissue repair. To test the hypothesis that CTGF is expressed in myocardial infarct tissue following acute myocardial infarction (AMI), we examined CTGF expression after AMI was experimentally induced in rats. Myocardial infarction was induced by left coronary artery ligation in male Sprague-Dawley rats. Northern blotting demonstrated that the CTGF mRNA expression on days 2, 7 and 14 was increased by 6-, 23- and 8-fold, respectively, compared to that in the pre-ligation hearts. In situ hybridization revealed CTGF mRNA signals on day 2 in myocytes in the infarct marginal zone and spindle-shaped mesenchymal cells (presumably myofibroblasts and fibroblasts) located between surviving myocytes in the infarct peripheral zone. On day 7, the signals were observed in the inner lesion of the infarct around infarct granulation tissue. Western blotting demonstrated that the CTGF protein expression on days 2, 7 and 14 was increased compared to the pre-ligation hearts. Immunopositive staining for CTGF was observed in the inner lesion of the infarct tissue on day 7. In conclusion, the findings demonstrated the increased expression of CTGF in the infarct tissue. Myocytes in the infarct marginal zone and spindle-shaped mesenchymal cells (presumably myofibroblasts and fibroblasts) were the cells responsible for CTGF production.