The primary and secondary neurons of the auditory and vestibular nerve of the mouse were examined for the presence of substance P (SP) by an immunocytochemical technique. Mouse heads were decalcificated with EDTA at 4°C and SP was visualized with the peroxidase-antiperoxidase technique. The results were as follows.1) No SP was detected in the spiral ganglion or vestibular nucleus.2) A very small amount of SP was found in some cell bodies of the vestibular ganglion and a moderate amount was found in many cell bodies of the ventralmost part of the ventral cochlear nucleus.