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1. A slow inward current in ventricular preparations of the dog heart can be measured by the voltage clamp method without interference from the initial rapid sodium current if the sodium system is inactivated by conditioning depolarization.

2. The slow inward current is very sensitive to variation in [Ca]_o. It occurs above the equilibrium potential of I_Na immediately after changing the bathing fluid to a sodium-free solution and persists under this condition for a long time without much alteration, while I_Na is rapidly abolished. Tetrodotoxin and [Mg]_o have no effect on this current component. These results strongly support the view that the slow inward current in cardiac tissue is carried by calcium ions.

3. The threshold for initiation of the calcium current is around -35 mV in Tyrode solution and is shifted to more negative potentials by either increasing [Ca]_o or reducing [Na]_o.

4. Calcium sensitive inward current tails associated with repolarization are assumed to represent a proportional measure of calcium conductance activated during the preceding depolarization. Calcium conductance declines rapidly with time in the inside negative potential range and slowly at positive potentials. The time constants for this `inactivation' process vary between 40 and 700 msec in the potential range -35 to +50 mV.

5. By using instantaneous current—voltage relations the reversal potential of calcium current was estimated to be about +60 mV in normal Tyrode solution. As shown in the Appendix, however, the calcium equilibrium potential cannot be considered to be constant.

6. The importance of the calcium current for the plateau of the cardiac action potential is discussed.

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