Perilymph is an extracellular fluid located within the cochlea (part of the ear) in 2 of its 3 compartments; the scala tympani and scala vestibuli. The ionic composition of perilymph is comparable to that of plasma and cerebrospinal fluid. The major cation of perilymph is sodium.
Perilymph vs. endolymph
The third compartment of the ear, the cochlear duct (or scala media) contains endolymph. Perilymph and endolymph have unique ionic compositions suited to their functions in regulating electrochemical impulses of hair cells. The electric potential of endolymph is ~80-90 mV more positive than perilymph.
It has also been suggested that perilymph and endolymph participate in a unidirectional flow that is interrupted in Meniere's disease.