The feasibility of small blood vessel occlusion by the argon laser was investigated on an animal experimental model, the vessels of the rat mesentery. Brief vascular occlusions could be achieved by producing endovascular thrombosis, mainly around altered erythrocytes. By summation of such individual effects, the duration of the occlusion could be decisively prolonged. This appears to be a way of delicately coagulating small vessels, i.e. with maintenance of vessel wall continuity.