Horakova et al. were the first to observe that the phase of late outward current carried by potassium ions in frog and squid nerve is virtually absent in voltage-clamped rat nodes of Ranvier. This observation has been recently confirmed by Chiu et al. in rabbit nodes of Ranvier, suggesting that the nodal membrane in the mammal generally has few if any potassium channels. The present voltage-clamp experiments show that large potassium currents can, however, be produced in normal rabbit nodes of Ranvier by acute treatment designed to loosen the myelin from the axonal membrane. From this we conclude that whereas potassium channels are absent in the mammalian nodal membrane, they are normally present in the internodal axonal membrane at least in the paranodal region.