Bucrylate was injected directly into the cerebral cortical arteries of mongrel dogs. Preparations for light and electron microscopy were obtained from 5 min to 5 months after the introduction of the polymer. A mixed pattern of damage to arterial endothelium was seen, including electron microscopic documentation of stripping away of the endothelium. Acute effects include a subocclusive thrombogenic matrix, which causes partial or complete thrombosis. The long-term reactions are those of a chronic inflammatory response to a foreign body.