A putative Na⁺ component playing a role in the initiation and maintenance of spontaneous discharge in Purkinje fibres was studied by means of the whole-cell patch-clamp technique in canine cardiac single Purkinje cells. In 4 mM [K⁺]_o, during depolarising clamp steps, a slowly inactivating current appeared at ~-58 mV, negative to the threshold for the fast Na⁺ current (I_Na; ~-50 mV). During depolarising ramps, the current underwent inward rectification with a negative slope region that began at ~-60 mV. The current underlying the negative slope increased during faster ramps, decreased as a function of time when the initial depolarising ramp was over, decreased during depolarisations positive to ~-35 mV and was much larger than the current during the symmetrical repolarising ramp. Increasing biphasic ('oscillatory') voltage ramps required much smaller currents at holding potential (V_h) -60 mV than at -80 mV and were associated with a marked decrease in slope conductance. At V_h -50/-40 mV, the oscillatory ramp currents and superimposed pulse currents reversed direction. The negative slope in the I-V relation as well as the change in current direction at -50/-40 mV were markedly reduced by tetrodotoxin (15 µM) and lidocaine (lignocaine, 100 µM) and therefore are due to a slowly inactivating Na⁺ current, labelled here I_Na3. Lower [K⁺]_o (2.7 mM) reduced the steady state slope conductance as well as the current in the diastolic range, and increased as well as shifted I_Na3 in a negative direction. High [K⁺]_o had the opposite effects. Cs⁺ (2 mM) and Ba²⁺ (2 mM) reduced the initial current during depolarising ramps but not I_Na3. In current-clamp mode, current-induced voltage oscillations elicited action potentials through a gradual transition between diastolic depolarisation and upstroke, consistent with the activation of I_Na3. Thus, the initiation and maintenance of spontaneous discharge in Purkinje strands appear to involve a voltage- and K⁺-dependent decrease in K⁺ conductance as well as the activation of a voltage- and time-dependent inward Na⁺ current (I_Na3) with slow inactivation kinetics.