1. In open chest dogs with complete heart block the ventricles were stimulated electrically at various steady rates, and a single premature or delayed beat was interposed to alter myocardial tension. This was followed by transient mechanical alternans. Steady alternans was induced by high stimulation frequencies.

2. It appeared that the recovery of the force of contraction of a beat could be approximated by an exponential function of the preceding time interval. We observed a correlation between the time-constant of this exponential curve and the force of contraction of the previous beat.

3. The results demonstrate that a general rule of mechanical alternans can be expressed by a single equation. This equation describes adequately (1) the duration of transient mechanical alternation produced by a delayed beat, (2) the production of steady alternation at rapid ventricular rates, and (3) the manifestations of post-extrasystolic potentiation during mechanical alternans.

4. Curves relating amplitude of one beat to the successive beat, based on experiments performed in in vivo dog hearts, corresponded to the results predicted by the equation.

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