Principal cells in the medial nucleus of the trapezoid body (MNTB) are believed to be critical components in the circuit subserving sound localization. These cells, located in the superior olivary complex, convert excitatory inputs, arriving from the contralateral cochlear nucleus by way of large somatic synapses (the calyces of Held), to inhibitory projections onto principal cells in the ipsilateral lateral superior olive (LSO). We have characterized a population of cells in the rat MNTB using intracellular recording and labeling techniques in a brain slice preparation. MNTB principal cells had spherical or ellipsoid somata that gave rise to single large-diameter dendrites, which branched extensively and often extended beyond the borders of MNTB. Commonly observed axonal projection targets included LSO, the superior paraolivary nucleus, and the medial superior olive, and occasionally the lateral nucleus of the trapezoid body. The projections of individual MNTB cells showed an orderly topography that is consistent with the known tonotopic maps of the nuclei. In response to current injection, principal cells exhibited several nonlinearities, including rectification for depolarizing currents and a “sag” in the membrane potential for hyperpolarizing currents. Superthreshold depolarizing currents elicited transient firing behavior. Application of the potassium channel blocker 4-aminopyridine reduced or eliminated the rectification in the current-voltage relationships and caused depolarizing currents to elicit repetitive firing. Stimulation of afferent inputs elicited short-latency spikes, presumably driven by calyceal synaptic inputs; long-latency, presumably polysynaptic, EPSPs; and short- and long-latency IPSPs. The duration of synaptic events was strongly dependent on membrane potential, and this effect was probably due to the intrinsic membrane properties of the cell. In all cases tested, EPSPs were blocked by CNQX or DNQX, and IPSPs were blocked by strychnine. Two injected non-principal cells differed from principal cells in their morphologies and physiological characteristics.