An ultrastructural study of the cerebral arteries obtained at autopsies on humans from infancy to old age was done. Intimal thickenings at bifurcations in infancy and childhood constantly showed various quantities of cellular debris that were seemingly dervied from the cellular constitutents of the wall, together with thickening, lamination, redundancy, and separation of basement membranes from the related cells. The extracellular lipid appeared to be derived from the transformation of cellular debris. Basement membrane changes indicated a serious disturbance in the relationship of mural cells to the connective tissues that was suggestive of interference with cohesion of the vessel wall. These morphological changes, hitherto overlooked, were more pronounced with age and in overt atherosclerosis, and were remakable similar to those induced experimentaly by hemodynamic stress.