The localization of glutamate immunoreactivity was examined in the guinea pig cochlea using affinity purified polyclonal antibodies to glutamate and immunoperoxidase post-embedding staining techniques on one micron plastic sections. Glutamate immunoreactive staining was seen in both inner and outer hair cells and in spiral ganglion cells and auditory nerve fibers. These results support the hypothesis that glutamate may function as the hair cell transmitter or as a precursor to the transmitter and add further support for an excitatory amino acid as the transmitter of the auditory nerve.