The onset of responsiveness to eighth nerve stimulation was examined in n. magnocellularis and n. laminaris, (second- and third-order neurons) of the chick brainstem auditory system. Extracellular microelectrode mapping techniques were used to examine postsynaptic responses in in vitro brainstem preparations. Two specific questions were addressed. First, what is the earliest time at which postsynaptic action potentials can be evoked in n. magnocellularis and n. laminaris by eighth nerve stimulation? Second, does responsiveness to eighth nerve stimulation develop along a spatial gradient in n. magnocellularis and, if so, how does this gradient compare with other developmental events observed in the chick auditory system? Postsynaptic responses in n. magnocellularis were first recorded at 11 days of incubation. Nucleus laminaris responses to direct stimulation of n. magnocellularis were also first recorded at 11 days, although n. laminaris responses to eight nerve stimulation were not seen until 12 days of incubation. A gradient of response development within n. magnocellularis was indicated by mapping of responsive sites on days 11-13. At 11 days, responses to eighth nerve stimulation were restricted to the most anteromedial portion of n. magnocellularis. Between 11 and 13 days, cells in increasingly more posterolateral portions of n. magnocellularis became responsive. This anteromedial-to-posterolateral gradient in n. magnocellularis is correlated with the basal-to-apical gradient of morphogenesis observed in the basilar papilla and morphogenetic gradients previously observed in n. magnocellularis and n. laminaris.