Objective: The aim was to test the hypothesis that cardiac angiotensin converting enzyme (ACE) is related to the accumulation of fibrous tissue in the heart. Methods: A model of tissue repair (pericardiotomy with left coronary artery ligation) was used, together with the following: quantitative in vitro autoradiography (125I-351A) to determine ACE binding density; immunohistochemistry (monoclonal ACE antibody, 9B9) to identify cells expressing ACE; and in situ hybridisation to localise cells expressing type I collagen mRNA. Age and sex matched rats receiving this operative procedure without subsequent infarction (sham operated) served as controls to those with left ventricular myocardial infarction. Serial heart sections obtained from each group at 3 days and at 1, 2, 4, and 8 weeks following operation were examined for morphological evidence of injury and inflammatory cells (haematoxylin/eosin) and fibrillar collagen (picrosirius red). Results: Following myocardial infarction: (a) on day 3, neutrophils and macrophages were present at the site of infarction, while fibrillar collagen and ACE binding were not increased compared with control; (b) at week 1, fibrillar collagen and ACE binding were present at the site of infarction and became progressively more advanced at 2, 4, and 8 weeks; (c) at week 2, there was increased ACE binding in the right ventricle and interventricular septum, when perivascular fibrosis of intramural coronary arteries and microscopic scars appeared, together with endomyocardial fibrosis of the septum; (d) there was marked ACE binding in the fibrosed visceral pericardium two weeks after operation in both myocardial infarction and sham operated groups; (e) there was type I collagen mRNA expression on postoperative week 1, localised within fibroblasts or fibroblast-like cells found at infarct and fibrous tissue sites in the right ventricle, septum, and pericardium; and (f) ACE-labelled cells included fibroblast-like cells, as well as macrophages and endothelial cells. Conclusions: Thus in this model of tissue repair, marked ACE binding density is associated with fibrillar collagen formation that appears within and remote to the site of myocardial infarction, including the pericardium. Cardiac ACE, originating from type I collagen producing cells, therefore represents an integral component of fibrous tissue formation in this rat model of tissue injury. Cardiovascular Research 1994;28:1423-1432