Fibronectin, basement membrane and type I collagen antigens have been localized in normal rat kidney by electron immunohistochemical methods. Immunoreactive fibronectin was found in the interstitial connective tissue matrix and on collagen fibers, while tubular, endothelial and smooth muscle basement membranes throughout the kidney were consistently negative. In the glomerulus immunoreactive fibronectin was abundant in the mesangial matrix. The peripheral glomerular basement membrane was occasionally reactive in a spotty, irregular manner. These findings suggest that fibronectin antigens are probably not a constituent of basement membranes. It is proposed that some fibronectin antigen may be trapped in the glomerular filter, and that normal glomerular cleansing mechanisms would transport this trapped fibronectin toward the mesangial areas where it would be eventually processed.