To investigate the role of internal elastic lamina in the formation of an aneurysm and the pathological progression of the aneurysmal wall and neck area, in a series of 32 adult rats, a small discrete mechanical lesion was made following an arteriotomy on the common carotid artery. Through the arteriotomy site, the lesion was made in the intima of the opposite side of the arterial wall by microsurgical technique at the proximal site to the bifurcation of the common carotid artery. Surgical sites of vessels were inspected grossly and light microscopically at intervals of 1, 4, 8 and 12 weeks(each group : 8 animals). Among the 32 animals, well defined aneurysms were seen in 15. In histological examination of the aneurysms, the internal elastic lamina almost disappeared or fragmented at the level of the aneurysmal neck. The aneurysmal wall was composed of only collagen fibers without evidence of elastic fibers. The aneurysmal wall and neck thickened progressively with collagen deposits. These aneurysms were similar to the human aneurysm histologically. This technique provides an experimental surgical formation of aneurysm and lends support to the important role of internal elastic lamina in the formation of an aneurysm. This experiment also showed progressiv thickening of the aneurysmal wall with collagen deposits.