Each of five cats (one congenitally deaf) had Pt-Ir electrodes placed in the modiolus. Some electrodes were stimulated with a 100 μamp, 0.25 msec balanced biphasic 50/sec pulse for 2 hours for a total of 20 to 40 hours each over periods of several months. Pre- and poststimulation measures of click-evoked N1 responses, averaged brain stem potentials, and impedances showed no long-term effects of damage to the nerves. Light microscopy showed very acceptable tissue tolerance and no evidence of damage caused by electrical stimulation. The deaf cat had about 10% residual nerve fibers which responded to stimulation.