Pyramidal cells in the dorsal cochlear nucleus (DCN) show three distinct temporal discharge patterns in response to sound: "pauser," "buildup," and "chopper." Similar discharge patterns are seen in vitro and depend on the voltage from which the cell is depolarized. It has been proposed that an inactivating A-type K+ current (IKI) might play a critical role in generating the three different patterns. In this study we examined the characteristics of transient currents in DCN pyramidal cells to evaluate this hypothesis. Morphologically identified pyramidal cells in rat brain slices (P11-P17) exhibited the three voltage-dependent discharge patterns. Two inactivating currents were present in outside-out patches from pyramidal cells: a rapidly inactivating (IKIF, tau approximately 11 msec) current insensitive to block by tetraethylammonium (TEA) and variably blocked by 4-aminopyridine (4-AP) with half-inactivation near -85 mV, and a slowly inactivating TEA- and 4-AP-sensitive current (IKIS, tau approximately 145 msec) with half-inactivation near -35 mV. Recovery from inactivation at 34 degrees C was described by a single exponential with a time constant of 10-30 msec, similar to the rate at which first spike latency increases with the duration of a hyperpolarizing prepulse. Acutely isolated cells also possessed a rapidly activating (<1 msec at 22 degrees C) transient current that activated near -45 mV and showed half-inactivation near -80 mV. A model demonstrated that the deinactivation of IKIF was correlated with the discharge patterns. Overall, the properties of the fast inactivating K+ current were consistent with their proposed role in shaping the discharge pattern of DCN pyramidal cells.