The effects of locally applied kainic acid on cells and fibers in the rat cochlea were examined in a quantitative and ultrastructural study. Doses of 5 nM per microliter of artificial perilymph destroyed part of the spiral ganglion type I cell population, with no ototoxic effects on cochlear hair cells or supporting cells. Type II cells also appeared unaffected. A quantitative evaluation of the cell loss with the 5 nM dosage showed that 34% of spiral ganglion neurons were lost 10 days after treatment. Doses of 20 nM per microliters and 40 nM per microliters did not result in increasing neuronal loss. This differential toxicity could reflect the presence of a sub-population of spiral ganglion cells with an increased number of KA receptors.