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1. The mechanism of oscillations at low membrane potentials in Purkinje fibres has been investigated using voltage clamp experiments.

2. The oscillations are generated by time-dependent variations in an outward current component, i_x1, that is activated over the voltage range -40 to 10 mV. During normal activity, this current is responsible for initiating full repolarization to the resting potential (Noble & Tsien, 1969b) so that the oscillations represent a failure of the normal repolarization process, probably as a consequence of a small change in background (leakage) current.

3. These oscillations are distinct from the normal pacemaker activity of Purkinje fibres which is generated by a separate time-dependent current, i_K2 (Noble & Tsien, 1968). i_K2 shows no time-dependence when the membrane potential variations are entirely positive to -65 mV and cannot, therefore, be involved in the oscillatory activity apart from contributing a background outward current.

4. The amplitude and frequency of the oscillations are very sensitive to applied currents less than 1 µA/cm². Larger currents abolish the oscillatory activity.

5. The mechanism of the oscillations is discussed in relation to the possible mechanisms underlying the natural pacemaker activity of the sino-atrial (SA) node.

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