Different classes of potassium (K+) outward current activated by depolarization were characterized in relay neurons acutely isolated from the rat lateral geniculate nucleus (LGN), using the whole-cell version of the patch-clamp technique. A fast-transient current (IA), activated at around –70 mV, declined rapidly with a voltage-dependent time constant (τ= 6 ms at + 45 mV), was 50% steady-state inactivated at –70 mV, and rapidly recovered from inactivation with a monoexponential time course (τ= 21 ms). IA was blocked by 4-aminopyridine (4-AP, 2–8 mM) and was relatively insensitive to tetraethylammonium (TEA, 2–10 mM). After elimination of IA by a conditioning prepulse (30 ms to –50 mV), a slow-transient K+ current could be studied in isolation, and was separated into three components, IKm, IKs and a calcium (Ca2+)-dependent current, IK[Ca]. The slow-transient current was not consistently affected by 4-AP (up to 8 mM), while TEA (2–10 mM) predominantly blocked IKs and IK[Ca]. The component IKm persisted in a solution containing TEA and 4-AP, activated at around –55 mV, declined monoexponentially during maintained depolarization (τ= 98 ms at +45 mV), was 50% inactivated at –39 mV, and recovered with τ= 128 ms from inactivation. IKs activated at a similar threshold, but declined much slower with τ= 2662 ms at +45 mV. Steady-state inactivation of IKs was half-maximal at –49 mV, and recovery from inactivation occurred relatively fast with τ= 116 ms. From these data and additional current-clamp recordings it is concluded that the K+ currents, due to their wide range of kinetics and dependence on membrane voltage or internal Ca2+ concentration, are capable of cooperatively controlling the firing threshold and of shaping the different states of electrophysiological behaviour in LGN relay cells.