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1. Membrane potential, tension and membrane current were simultaneously recorded from frog ventricular strips in a modified sucrose-gap which enabled control of membrane potential by voltage clamp.

2. Shortening the frog ventricular action potential by repolarizing the membrane to the resting potential terminates contraction.

3. Depolarization to the level of the normal action potential plateau for longer than about 80-100 msec (up to 30 sec) produces and maintains tension for the duration of the depolarization.

4. Depolarizations less than about 80 msec in duration generate no tension but can facilitate the tension response to subsequent depolarizations. The facilitating effect of a short depolarizing pulse persists for no longer than 0·5 sec.

5. The mechanical threshold is about -50 mV; the relation between membrane potential and tension is fairly linear from about +5 to +80 mV.

6. Variation of holding potential, below the mechanical threshold, has no effect on the tension—voltage relation. The absolute membrane potential rather than pulse amplitude determines the developed tension.

7. Increasing external calcium increases the slope of the voltage—tension relation.

8. Contraction of the frog ventricle is directly controlled by the electrical activity of the surface membrane.

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