TGF-β1 has been examined in the heart during myocardial infarction caused by ligation of the left coronary artery. Infracted and uninfected myocardium have been compared by immunohistochemical staining of TGF-β1 and by Northern blot analysis of mRNA. Normal ventricular myocytes are strongly stained by an antibody to TGF-β1. Progressive loss of staining of these myocytes begins within 1 hr after coronary ligation. However, by 24-48 hr after ligation, intense staining of myocytes at the margin of infracted areas is seen. Northern blots of infracted myocardium 48 hr after ligation show a 3to 4-fold increase in the principal 2.4 kb TGF-β1 mRNA; there is also a marked increase in a minor 1.9 kb transcript. In the same tissue samples, there is a 2-fold decrease in the mRNA for the glycolytic enzyme, glyceraldehyde-3-phosphate dehydrogenase. The results indicate a significant role for TGF-β in the response of the heart to injury.