1. Large myelinated nerve fibres were isolated from rats and the membrane action potential was recorded in single nodes of Ranvier. Potential clamp experiments were performed at 24 degrees C and in one fibre also at 33 degrees C. 2. Positive potential steps were associated with an initial, mainly Na carried, current with an equilibrium potential (Ue) of 40 +/- 13 mV. The INa vs. U curve showed rectification at large positive potentials. 3. The Na permeability (PNa) curve was characteristically S-shaped with a half value at -48 +/- 6 mV and a ceiling value of 3.7 +/- 0.7 cm.sec-1.10(-3). PNa as defined by the constant field equation accounted for the rectification of the INa/U curve. 4. The potential and time dependence of the rapid inactivation (h) process was described quantitatively in terms of its steady state (h infinity) vs. U curve and its rate constants (alpha h and beta h) vs. U curves. h infinity (U = -80 mV) was 0.68 +/- 0.09. 5. The delayed currents were small and the ionic specificity of the delayed permeability changes was not identified. Tentatively calculated PK at large potentials was 0.21 +/- 0.06 cm.sec-1.10(-3). The leak conductance (gL) was 130 +/- 33 mS.cm-2. 6. A temperature rise in one experiment from 24 to 33 degrees C increased the rate constants (alpha h and beta h), but did not significantly change the size of the delayed currents. 7. INa was completely blocked by 25 nM-TTX in the outside solution. External application of 5 mM-TEA only slowly and incompletely blocked the small delayed currents. This effect was not fully reversible. 8. A comparison with frog fibres showed that the node in rat fibres had (a) a lower Ue of the initial current (probably because of a lower Na selectivity of the channel for the initial current), (b) approximately equal max. peak PNa, (c) about 10 mV negatively shifted PNa vs. U and h infinity vs. U curves, (d) quantitatively similar relations between rate constants of inactivation (alpha h and beta h) and U at 20--24 degrees C, (e) PK that was 1/5 of PK in frog fibres, (f) 4--5 times larger leak conductance, (g) membrane action potentials of smiliar amplitude and duration.