(1) Action potentials and membrane currents were recorded in single myelinated rat nerve fibres at 20 and 37 degrees C. Three experiments were also performed in single cat nerve fibres. (2) K currents were blocked by internal CsCl and external TEA. The steady state and kinetic parameters of Na activation and inactivation were determined at both temperatures. (3) When the temperature was raised from 20 to 37 degrees C, steady state Na activation, m infinity (V), and inactivation, h infinity (V), did not change significantly. (4) The time constant of Na activation, tau m, was determined within the potential range of -40 to 125 mV at 20 degrees C and V = 40-60 mV at 37 degrees C. The temperature coefficient, Q10, of tau m was 2.2. (5) The decay in the Na current was described by two exponentials at both temperatures. The amplitude of the slow phase was 1-10%. The time constant of the fast phase of Na inactivation, tau h1, was determined at both temperatures within the potential range of -50 mV to 125 mV. The Q10 of tau h1 was 2.9 and did not depend on potential. (6) The Na equilibrium potential was 152 mV at 20 degrees C and 144 mV at 37 degrees C. The leakage conductance was 24 nS at 20 degrees C and 43 nS at 37 degrees C. These differences were interpreted as signs of fibre deterioration at higher temperature. (7) The results from the current and voltage clamp experiments performed in the cat nerve were essentially the same as those in the rat nerve fibres.(ABSTRACT TRUNCATED AT 250 WORDS)