Auditory nerve fibers carry impulses from the cochlea to the cochlear nuclei. There the temporal firing patterns of auditory nerve fibers are preserved by some cells and altered by others. The two factors which govern how firing patterns are shaped are (1) the intrinsic electrical properties of cells that determine the size and time course of voltage changes caused by synaptic currents and (2) the synaptic circuitry between cells. The electrical properties of cells were measured by recording the responses to current injected intracellularly into brain slice preparations. The synaptic responses to electrical shocks of the auditory nerve were used to determine the functional properties of synaptic connections. The three distinct types of electrical properties of cells that can be distinguished electrophysiologically in similar preparations of mature tissue, bushy and stellate cells in the ventral cochlear nucleus [(1984) J. Neurosci. 4, 1577-1588] and cells in the dorsal cochlear nucleus [Hirsch and Oertel (1987) (submitted); Oertel et al. (1987) In: Functions of the Auditory System, Editor: S. Hassler. J. Wiley and Sons (in press)] can be differentiated at least as early as 7 days after birth. Young cells, however, have higher input resistances and lower input capacitances than mature cells, and they cannot sustain high firing rates. Bushy and stellate cells in the ventral cochlear nucleus respond to electrical stimulation of the auditory nerve with both excitatory and inhibitory postsynaptic potentials as early as 4 days after birth. The synaptic potentials occur with longer and more variable latencies than in mature cells and synapses fatigue more easily, however. Cells of the dorsal cochlear nucleus also receive both excitatory and inhibitory synaptic inputs 4 days after birth, upon stimulation of the auditory nerve. No systematic changes were detected in these synaptic responses as a function of age but this may have been because the variability in the shape and timing of synaptic responses was large even in mature tissue.