Principal neurones of the mouse medial nucleus of the trapezoid body (MNTB) possess multiple voltage-gated potassium currents, including a transient outward current (or A-current), which is characterized here. The A-current exhibited rapid voltage-dependent inactivation and was half inactivated at resting membrane potentials. Following a hyperpolarizing pre-pulse to remove inactivation, the peak transient current was 1.07 nA at -17 mV. The pharmacological characteristics of this A-current were consistent with Kv4 subunits in expression studies; the A-current was resistant to block by tetraethylammonium and dendrotoxin-I but sensitive to millimolar concentrations of 4-aminopyridine and 5 microM hanatoxin. Immunohistochemistry confirmed that Kv4.3 sub-units are present in the MNTB. In a single-compartment model of an MNTB neurone, the A-current served to accelerate the decay of the initial action potentials in a stimulus train and suggested that removal of A-current steady-state inactivation could raise firing threshold for non-calyceal synaptic inputs. This A-type current was not observed in the rat.