By extracting the a. carotis communis and the a. carotis interna of rabbit, frozen sections of about 30μ in thickness were prepared. These were immersed in diluted causatic soda solution, and the conditions of the fenestration of elastic membrane were studied. On the other hand, some of the above specimens were fixed with 10% formalin solution, and after staining elastic fibers they were observed under light microscope as the control. In addition, they were also observed electron microscopically. In the internal elastic membranes of both arterial walls in the case of the ones immersed in causatic soda solution for several days, the surface area of the fenestration was about 15-20% which is mch more than the area of 2-5% seen in the section specimens, and their size ranged 2.5-10μ and most of them were about 6μ and the morphology revealed that the fenestrations shift from a cross-wise elliptic shape to a longitudinal elliptic shape from the inner layer to the middle layer. The above-mentioned fenestration is not a simple space but it contains elastic fibers crossing longitudinally and cross-wise within it.