The role of positive dynamic current (Cl- current) on action potentials of [human] cardiac Purkinje fibers was studied. The removal of most extracellular Cl- brought about a slowing of the repolarization process. The most prominent effect was noted on the initial rapid repolarization (phase 1) of the action potentials. Purkinje action potentials showed marked and consistent slowing of phase 1 with increasing rate of stimulation. The of [Cl-]o caused marked loss of these frequency dependency in phase 1 and this effect was more prominent with slow rate of stimulation. The slopes of phase 2 and phase 3 also changed with varying frequency of stimulations but these changes were not affected much by Cl- removal. Cl- conductance at the peak of the positive dynamic current increased linearly with membrane voltages from -20 mV up to +20 mV. Above this voltage, the conductance reached a plateau. A steady-state current voltage relationship was not influenced much by the Cl- removal except slight decrease (31%) in membrane conductance near the resting membrane potential. The increase in frequency of pulses produced marked decrease in the positive dynamic current in the voltage clamp experiments. The positive dynamic current mainly contribute to the electrogenesis of phase 1 in Purkinje action potentials.