The fenestrated blood vessels surrounding the endolymphatic sac in guinea pigs were investigated with the help of freeze-fracturing. The technique exposes the structure and distribution of vascular pores as well as interendothelial bridges or tight junctions between vascular endothelial cells. It is possible to get a three-dimensional comprehension of the vascular structure which can be compared with that of conventional transmission electron microscopy (TEM). Discontinuity in the junctional elements as seen in some endothelial layers and the high number of fenestrations organized in geometric patterns, as well as the abundant, randomly distributed micropinocytotic vesicles seem to bear out the theory that the endolymphatic sac is one of the most metabolically active parts of the inner ear and may be involved in the turnover of endolymph.