Light microscopic immunohistochemical studies were performed to evaluate the distribution of fibronectin in paraffin sections of p-formaldehyde-fixed normal rat hearts and the hearts of rats that had undergone ligation of the left coronary artery. A peroxidase-labeled antibody technique was used, together with appropriate immunohistochemical control procedures, for the localization of fibronectin in normal hearts and in the hearts of sham-operated animals. Fibronectin was localized in the interstitial space between myocytes, and beneath arterial, venous, and capillary endothelium. At 4 hours after coronary ligation, fibronectin was localized in a patchy fashion in the cytoplasm and interstitial space of some of the myocytes in the area supplied by the ligated vessel. At 24 hours, there was more intense, homogeneous staining in necrotic myocytes in the infarcted area and in the capillary endothelium in the border zone. At 48 hours, the intensity of staining for fibronectin was maximal in and between the necrotic myocytes in the center of the infarct and in proliferating and migrating capillaries and fibroblasts in the border zone. Similar patterns of localization were observed at 3 and 7 days after coronary ligation, but with progressive decreases in the intensity of staining. Two sources of fibronectin appeared to have contributed to these changes: plasma fibronectin diffusing through damaged blood vessels would account for the early staining observed in necrotic myocytes in the center of the infarct, whereas de novo synthesis of fibronectin by connective tissue cells and endothelial cells in sprouting capillaries would be responsible for the subsequent staining observed in viable capillaries in the border zone of the infarct. Known properties of fibronectin in vitro, combined with these in vivo observations, indicate that fibronectin may influence the thrombotic, inflammatory, angiogenic, and fibrotic processes involved in infarct healing.