A persistent outward K+ current (IPO), activated by depolarization from resting potential, has been identified and characterized in rat subthalamic nucleus (SThN) neurones using whole-cell voltage-clamp recording in brain slices. IPO both rapidly activated (tau = 8 ms at +5 mV) and deactivated (tau = 2 ms at -68 mV), while showing little inactivation. Tail current reversal potentials varied with extracellular K+ concentration in a Nernstian manner. Intracellular Cs+ did not alter either IPO amplitude or the voltage dependence of activation, but blocked transient (A-like) outward currents activated by depolarization. When extracellular K+ was replaced with Cs+, IPO tail current reversal potentials were dependent upon the extracellular Cs+ concentration, indicating an ability to conduct Cs+, as well as K+. IPO was blocked by Ba2+ (1 mM), 4-aminopyridine (1 mM) and tetraethylammonium (TEA; 20 mM), with an IC50 for TEA of 0.39 mM. The IPO conductance appeared maximal (38 nS) at around +27 mV, half-maximal at -13 mV, with the threshold for activation at around -38 mV. TEA (1 mM) blocked the action potential after-hyperpolarization and permitted accommodation of action potential firing at frequencies greater than around 200 Hz. We conclude that IPO, which shares many characteristics of currents attributable to Kv3.1 K+ channels, enables high-frequency spike trains in SThN neurones.