1. A study has been made of the ionic currents in voltage-clamped single rabbit nodes of Ranvier at 22-26 degrees C both under normal conditions, and after the nerve fibres had been acutely demyelinated by a variety of treatments designed to lossen the myelin from the axonal membrane. 2. The myelin-loosening treatments included application of various combinations of: lysolecithin (to dissolve the myelin); collagenase (to loosen the connective tissue in the nodal region); high-potassium Locke solution, hypertonic and hypotonic solutions (to induce axonal volume changes). 3. At a critical stage in such treatment (usually after 15-45 min) a large outward current suddenly appeared. 4. There was no substantial change in the size of the measured inward sodium current when measured at this critical stage. 5. The outward current was blocked by internal TEA and caesium ions, had a reversal potential that became more positive when the external potassium concentration was increased, was kinetically similar to the known potassium current in frog fibres, and was therefore assumed to be a potassium current. 6. The phase of large outward current, whenever it appeared, was always accompanied by the appearance of a slow transient capacitative component in the leakage current, which indicated a marked increase in the effective nodal capacity (of 10- to 60-fold). We suggest that the slow transient capacity current reflected charging of newly exposed axonal membrane, probably in the paranodal region, which was uncovered by the various acute demyelination treatments. This internodal membrane seems to contain mostly potassium channels and few, if any, sodium channels. 7. Newly dissected fibres occasionally showed large potassium currents before treatment, particularly if they were deliberately stretched during dissection; a marked slow capacity transient current was consistently present in these fibres. 8. The effects of acute paranodal demyelination on the sodium and potassium currents, and on the transient capacity currents, can be simulated by a model in which the node is coupled to a cable-like paranode which contains Hodgkin--Huxley type potassium channels and which has a much higher leakage resistance. 9. The functional significance of the presence of potassium channels in rhe internodal region (at least in the paranode) of mammalian fibres is discussed.