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1. Membrane ionic current-voltage (I-V) relations of the frog ventricular myocardium were measured during the action potential with a new single sucrose gap voltage clamp technique. 2. The I-V relation is linear during the plateau and rapid repolarization phases of the action potential and during the development of the regenerative threshold of repolarization. 3. Time dependent I-V relation during a series of voltage clamp pulses of clamp initiation. 4. The membrane conductance is remarkably constant during the plateau and is about 85 mumhos/muF of membrane capacitance. 5. Chloride conductance is about 18% of the total ionic conductance during the plateau and is not time dependent. Two inward Cl-movement during a normal action potential is sufficient to approximately halve the action potential duration. 6. Membrane conductance did not change significantly when Ca2+ was omitted from the bathing medium. 7. Epinephrine increased the duration of the action potential and the total ionic conductance during the platiau in normal and Ca-free media. 8. Separation of Na+ and K+ currents in muscles bathed in 'zero' Ca2+, 'zero' Cl- solution indicates that the inward and outward currents are balanced to within 2% during the slow repolarization. 9. The results indicates that a fine balance between conductance changes cardiac action potential. The possibility of a cross ionic interaction in the heart cell membrane is suggested.

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