In order to clarify the cause of arterial changes after intra-arterial infusion of anti-cancer drugs (AI-AD) experiments were made on the arteries of rabbits. Histopathologic sections, 7 days after AI-AD revealed endothelial damage characterized by pyknosis, hyalinization with edematous change of the intimal layer. Proliferation of the endothelial cells was also observed. The cause of narrowing or occlusion of the arteries was considered to occur with thrombus formation surrounded by the proliferated endothelial cells and this suggested that thrombolytic agents would be effective to prevent these arterial changes.