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1. The mechanical and electrophysiological effects of Sr were evaluated and compared to those of Ca in isolated, electrically driven toad ventricular muscle strips. The effects of Ca and Sr were compared at concentrations from control (2 mM) to 10 mM either by substitution of Sr for Ca at equimolar concentration, or by maintenance of a constant total Ca plus Sr concentration within which individual Ca and Sr concentrations were varied. 2. Changes in the degree of contractile activation were evaluated in terms of changes in maximal dT/dt of isometric contractions, maximal dL/dt of very lightly loaded isotonic contractions, and the shape of after-loaded force-velocity curves, with specific attention directed to the shape of the curves as they approached Vmax on the velocity axis. Effects on the cell membrane were evaluated in terms of changes in the transmembrane action potential (recorded with glass micro-electrodes) and in the mechanical parameters directly related to its duration in amphibian ventricle, viz. the duration of isometric tension and of isotonic shortening. Isometric tension and action potentials were recorded simultaneously. 3. Ca and Sr, at concentrations above control, had similar but not identical effects on dT/dt and dL/dt. Both ions alone in equimolar concentrations, or together at constant total Ca plus Sr concentration, increased dT/dt and dL/dt and shifted force-velocity curves upward. At constant total Ca plus Sr concentration, force-velocity curves were virtually superimposable as they approached the velocity intercept. 4. The duration of the action potential was markedly prolonged by Sr and shortened by Ca in concentrations above control. Unlike dT/dt and dL/dt, the total duration of isometric tension and isotonic shortening depended upon the specific Ca and Sr concentrations within a constant total concentration, and were progressively prolonged as the Sr concentration was increased. 5. The similar effects of Ca and Sr on dT/dt, dL/dt, and on the force-velocity relationship at light loads depended upon the presence of Ca ions. In Sr alone, dT/dt and dL/dt were faster than in an equimolar concentration of Ca, and time to maximal dT/dt and dL/dt was prolonged. The force-velocity curve in Sr alone was consistently shifted upward beyond the other curves in which Ca was present. These differences between the two ions are attributed in part to the rapid and early repolarization of the action potential in elevated Ca and the resultant abbreviation of the build up in active state and slower dT/dt and dL/dt. 6. The results suggest that Ca and Sr act in a similar although not identical way in activating contraction but are competitive at the cell membrane.