Abstract Thrombolysis was studied in dog coronary arteries after reperfusion by i.v. streptokinase or tissue plasminogen activator (t-PA). A thrombus was formed with whole blood and thrombin in the left anterior descending coronary artery of anesthetised, open-chest dogs. Reperfusion was monitored by electromagnetic flow probe. Arteries from sham-operated dogs (no occlusive thrombi) and from saline, streptokinase and t-PA-infused dogs (collected at the time of reperfusion or 40 min after reperfusion) were examined by scanning electron microscopy. The thrombus in saline-infused dogs was composed of closely packed red blood cells (rbc) and areas rich in fibrin and platelets. Arteries collected from streptokinase- and t-PA-infused dogs at reperfusion showed that flow returned through irregular channels that meander over and delve through the clot. Reperfusion channels were not seen in saline-infused dogs. Despite return of blood flow to control levels, a portion of the thrombus remained in the lumen of the artery in both streptokinase- and t-PA-infused dogs. Deposition of rbc, platelets and fibrin occurred on the surface of the residual thrombus and the adjacent arterial endothelium. These findings indicate that partially occlusive thrombi can remain after successful thrombolysis and may serve as substrata for further deposition of fibrin and platelets.