1. Principal cells in the medial superior olive (MSO) receive low-frequency information from both ears via left and right cochlear nuclei. In vivo extracellular records suggest that some MSO neurons respond optimally only when the binaural acoustic signal has a precise interaural delay. Thus MSO cells, in particular principal cells, are thought to be the first stage in the processing of interaural time difference cues that provides information as to the location of a low-frequency sound in space. 2. Despite this proposed fundamental role for the MSO, certain features of this nucleus make in vivo recordings from any cell type here very difficult to obtain. Only a small number of extracellular records and no intracellular recordings are reported in the literature. Using sharp, neurobiotin-filled glass electrodes to record intracellularly from cells in an in vitro brain slice of the guinea pig superior olivary complex, I have begun to assess the anatomic and physiological features of cells in the MSO that might be relevant to such a functional role in vivo. 3. Two basic MSO cell types, designated principal and nonprincipal, could be distinguished on the basis of certain anatomic and physiological differences. 4. Labeled principal cell bodies were located at all dorsoventral location within the MSO. Labeled nonprincipal cells were located in or around the dorsal aspects of the nucleus. Principal cells typically had thick bipolar dendrites (1 directed medially, 1 laterally) that did not taper or branch significantly except at their terminations. Nonprincipal cells were multipolar with three to nine thinner primary dendrites that did not branch preferentially in a mediolateral direction. Principal cell axons gave off collaterals terminating in and around the dorsal MSO. Nonprincipal cells also had axon in and around the dorsal MSO. Nonprincipal cells also had axon collateral branches innervating dorsal MSO, but these axons could branch more extensively and project further down the dorsoventral aspect of the nucleus. 5. Principal cells typically responded to depolarizing current pulses with one or a few spikes at current onset. When bathed in saline containing 4-aminopyridine (4-AP), they fired repetitively to the same depolarizing current pulses. This would indicate a depolarization-induced nonlinearity similar to that seen in principal cell types of two other auditory brain stem nuclei, the anteroventral cochlear nucleus and medial nucleus of the trapezoid body. Nonprincipal cells normally fired repetitively to depolarizing current pulses even close to spike threshold. Both cell types could show a sag in the membrane potential to hyperpolarizing current pulses.(ABSTRACT TRUNCATED AT 400 WORDS)