Healing of the carotid artery after endarterectomy in adult mongrel dogs was investigated at intervals from 15 minutes to two weeks after surgery by light and scanning electron microscopy. In the control group, a fibrin-platelet carpet was found on the surface of the endarterectomized wall of the vessel soon after surgery. A large, typical, red thrombus was noted on the endarterectomized surface in three of six control animals 15 minutes after the operation, and in another one hour after the operation. On Day 1 after surgery, no new thrombus formation was detectable. On Day 3, most of the mural thrombi had disappeared, and formation of new-endothelium had started and was completed about one week after the operation. In the group treated with aspirin, thrombus formation in the wall was much less just after operation, and no large thrombi were seen in any animals. Almost no newly formed endothelium was detectable on Day 3. After one week, new endothelium covered only about 60% of the inner surface of the artery, but after two weeks the formation of new-endothelium was almost complete in all dogs. Thus, with aspirin treatment, formation of new endothelium seemed to be delayed and the new endothelial cells seemed to grow somewhat differently from those in the control group.