The responses of neurons in the lateral superior olive (LSO) and medial nucleus of the trapezoid body (MNTB) to repeated electrical stimulation of the trapezoid body were investigated in a brain slice preparation of the mouse superior olivary complex. Brain slices, 400-500 microns thick, were cut in the frontal plane and were maintained for physiological recording in a bath of warm, oxygenated saline. Both intracellular and extracellular recordings were made with glass micropipettes filled with 4 M potassium acetate. Bipolar stimulating electrodes were placed on the trapezoid body ipsilateral and contralateral to the superior olive. Current levels were set so that an ipsilateral pulse elicited a single action potential in LSO and a contralateral pulse elicited a single action potential in MNTB. Trapezoid body fibers were then stimulated with trains consisting of 10 current pulses. Repeated stimulation at low rates resulted in a number of spikes equal to the number of current pulses. Pulse rate was then increased to determine the limits of response to repeated stimulation. In the MNTB, neurons were capable of following very high rates of stimulation without reduction in response probability. On the average, MNTB neurons responded with a probability of 0.9 at 667 Hz. In contrast, most LSO neurons were incapable of following high rates of stimulation. LSO neurons responded with a probability of 0.9 at 125 Hz. For some LSO neurons, application of strychnine (0.5 microM) to the bath increased the probability of firing at high rates. MNTB neurons were unaffected by strychnine.