Cochleas of adult guinea pigs and rats, and 6-day-old rat pups, were injected, through the round window, with 2 microliters of artificial classical Konishi perilymph containing 1 nmol kainic acid (KA). 5 min later, they were fixed, removed, and processed for electron microscopy. In all KA-treated cochleas, the injection resulted in a severe swelling of auditory dendrites below the inner hair cells (IHCs). Below the outer hair cells (OHCs), the swelling appeared only in the 6-day-old rats, not in adult animals. These results are significant in three different ways: (1) They confirm the strong difference between afferents innervating the IHCs and the OHCs in adult cochleas. (2) They shed some light on the synaptic plasticity found at the OHC level during synaptogenesis. (3) They support the hypothesis that glutamate, or a related substance, is the IHC neurotransmitter.