1. Double barrel potassium-sensitive micro-electrodes were used to measure fluctuations in extracellular potassium ion concentration in large automatic canine Purkinje fibres.2. Slow accumulations of potassium were seen in the extracellular space during prolonged beating. Following cessation of prolonged beating, a depletion of extracellular potassium ions was noted.3. The time course of these slow changes in extracellular potassium concentration were shown to be a function of the diffusion properties of the preparation, and the rate and degree of activation of the sodium pump.4. Action potential duration and maximum diastolic potential were simultaneously monitored during and after these periods of rapid stimulation, using conventional intracellular micro-electrodes.5. Initial depolarization and later hyperpolarization of the maximum diastolic potential appeared to occur as a direct result of changes in extracellular potassium concentration and level of pump activation induced by the sudden and prolonged alteration in stimulus rate.6. Following prolonged stimulation, dramatic changes in the automatic beating rate were correlated with changes in extracellular potassium and pump rate, and their effects on the maximum diastolic potential and other parameters of the diastolic potential depolarization.7. For some locations of the potassium-sensitive electrode tip, large fluctuations in extracellular potassium were seen during single beats. Alterations in action potential duration during abrupt rate changes appear to derive in part from modulation by these extracellular potassium concentration fluctuations altering net membrane currents. Slower shifts in action potential duration appear correlated to the degree of sodium pump activation.