1. We investigated voltage-gated potassium channels in human peripheral myelinated axons; apart from the I, S and F channels already described in amphibian and rat axons, we identified at least two other channel types. 2. The I channel activated between -70 and -40 mV, and inactivated very slowly (time constant 13.1 s at -40 mV). It had two gating modes: the dominant ('noisy') mode had a conductance of 30 pS (inward current, symmetrical 155 mM K+) and a deactivation time constant (tau) of 25 ms (-80 mV); it accounted for most ( approximately 50-75 %) of the macroscopic K+ current in large patches. The secondary ('flickery') gating mode had a conductance of 22 pS, and showed bi-exponential deactivation (tau = 16 and 102 ms -80 11 mV); it contributed part of the slow macroscopic K+ current. 3. The I channel current was blocked by 1 microM alpha-dendrotoxin (DTX); we also observed two other DTX-sensitive K+ channel types (40 pS and 25 pS). The S and F channels were not blocked by 1 microM DTX. 4. The conductance of the S channel was 7-10 pS, and it activated at slightly more negative potentials than the I channel; its deactivation was slow (tau = 41.7 ms at -100 mV). It contributed a second component of the slow macroscopic K+ current. 5. The F channel had a conductance of 50 pS; it activated at potentials between -40 and +40 V, deactivated very rapidly (tau = 1.4 ms at -100 mV), and inactivated rapidly (tau = 62 ms at +80 mV). It accounted for the fast-deactivating macroscopic K+ current and partly for fast K+ current inactivation. 6. We conclude that human and rat axonal K+ channels are closely similar, but that the correspondence between K+ channel types and the macroscopic currents usually attributed to them is only partial. At least five channel types exist, and their characteristics overlap to a considerable extent.