Voltage clamp experiments on isolated sheep Purkinje fibres showed an increase of the steady state outward membrane current, over the potential range -65mV to -15 mV, in the presence of tetrodotoxin (TTX, 3.10(-5 M). This "window" current is considered to be the steady state component of the fast sodium current (INa), resulting from the crossover of the activation and inactivation curves which govern the opening of the sodium channel. TTX had no significant effect on the reversal potential, activation curve, kinetics or instantaneous I-V relationship of the pacemaker current IK2. The window found in these experiments extends to potentials well into the range of the action potential plateau. Consequently small changes of the steady state INa might have large effects on the action potential duration. The effects of TTX and local anaesthetics are discussed in this context.