The fine structure of tissue from 6 berry aneurysms, 7 saccular aneurysms and 2 fusiform aneurysms produced by microvascular surgery in rabbits was examined. The rabbits were maintained on a stock diet and sacrificed from 3 mth to 3 yr 10 mth postoperatively. The aneurysmal walls exhibited thickening of the intimal type with progressive destruction of normal architecture and degenerative and necrotic changes in the smooth muscle cells. Thickening, lamination, redundancy, reticulation and separation of basement membranes about muscle cells and an abundance of matrix vesicles and cell debris were prominent features. In several aneurysms, extracellular lipid, cholesterol clefts and lipophages were observed and fibrin was found in 4 aneurysmal walls. These vascular lesions evinced remarkable similarity to the degenerative changes in experimental arteriovenous aneurysms, human cerebral aneurysms and spontaneous atherosclerosis.