Percutaneous coronary transluminal angioplasty is currently being proposed as a possible treatment for some patients with significant coronary arterial lesions. In this study scanning electron microscopy was used to assess the morphologic changes caused by the angioplasty balloon catheter in normal canine coronary arteries. Balloon inflation uniformly caused endothelial denudation with virtually immediate dense platelet adhesion to the subendothelial matrix. This platelet adhesion was unaffected by pretreatment with aspirin (35 mg/kg body weight) or heparin (2,000 units). However, intravenous administration of low molecular weight dextran almost completely prevented platelet adhesion to the exposed subendothelial connective tissues. It is concluded that even in normal canine coronary arteries (1) loss of endothelium is seen after transluminal angioplasty; (2) this endothelial damage stimulates platelet adhesion and thrombus formation; and (3) the immediate layering of platelets can be prevented by administration of low molecular weight dextran.