Freeze fracture replicas of the guinea pig inner ear were studied under the electron microscope to define the blood-perilymph barrier morphologically. This barrier is represented basically by the continuous endothelium of the inner ear capillaries. The endothelial cells contain only a few micropinocytotic vesicles in contrast to those of the stria vascularis vessels. The cochlear plexus also exhibits some special differences. Tight junctions of the continuous mesothelial type connect the endothelial cells of the inner ear capillaries. Cell membranes of these cells possess a lower particle density than those of the stria vascularis vessels. In general, the blood-perilymph barrier is morphologically similar to the blood-brain barrier.