1. The effects of toxin II from scorpion Androctonus australis Hector (AaH II) on the Na current of frog myelinated nerve fibres were analysed under voltage-clamp conditions. 2. Like other alpha-scorpion toxins and Anemonia toxin II, AaH II both increased the inactivation time constants of peak Na current and induced a non-inactivatable Na current (maintained current). 3. In the presence of AaH II, the slope of the maintained conductance-voltage curve was less steep than that corresponding to the peak conductance and the maintained current reversed at a voltage about 20 mV more negative than the peak current. 4. When the peak current was inactivated by pre-depolarizations, 'on' and 'off' relaxation kinetics of the maintained current were an exponential function whose time constant changed with voltage in a bell-shaped manner. At 0 mV, the time constant was about 10 ms. 5. The effects of AaH II could be decomposed into fast effects (increase in inactivation time constants of the peak current) which developed within about 5 s and slow effects (increase in maintained current and changes in initial amplitudes of fast and slow phases of peak current inactivation) which developed within about 30 s. 6. These two types of AaH II effects could be completely removed by conditioning depolarizations giving rise to outward currents. 7. A model is proposed in which the binding of the toxin with its receptor is modulated by membrane potential and internal cations, the appearance of the maintained current is modulated by the environment of channels and the change in inactivation time constants is modulated by membrane potential. The maintained current would correspond to the transformation of a fraction of channels into a non-inactivable (late) form.