1. Sodium currents were measured under voltage-clamp conditions in Ranvier nodes of rat and frog nerve fibres at 20 degrees C. Voltage errors due to the resistance in series with the nodal membrane were minimized by reducing sodium currents with tetrodotoxin in the extracellular solutions. 2. The stationary and kinetic properties of sodium activation and inactivation were determined for a wide range of potentials (V) from -40 to 160 mV with respect to the initial holding level (V = 0 mV). 3. The curves m infinity(V) and h infinity(V) of stationary sodium activation and inactivation were not different in rat and frog fibres. 4. The time constants tau m, tau h of sodium activation and inactivation were normally larger in the rat than in the frog. At moderate depolarizations (0 less than or equal to V less than or equal to 80 mV) tau m in the rat was 15-50% larger; the ratio of the rat to the frog tau h values was usually smaller. Thus tau m/tau h = 0.116 for the rat and 0.0965 for the frog at V = 60 mV (potential with maximum peak sodium inward current). 5. Sodium inactivation in rat nerve was slowed and became incomplete by application of intra-axonal iodate or by treatment with external Anemonia toxin II (ATX II), chloramine-T or Ruthenium Red. Peak sodium currents were not increased by these substances. 6. Wash-out of ATX II from frog nerve was rapid and complete but partly irreversible in rat nerve. This suggests different properties or accessibilities of sodium channels in frog and rat nodes.