The influence of temperature, contractures, and CA²⁺ on the processes of healing over and muscle contraction was investigated in toad myocardium by simultaneously measuring the size of the injury potential and the twitch tension. Stimulation with single pulses and electrically induced contractures of short duration (2-3 seconds) markedly increased the rate of healing, and epinephrine (10⁻⁶ g/ml) increased the effectiveness of stimulation. Other results supported the view that the healing process was largely dependent on temperature, since at 15°C or 10°C the rate of healing over was greatly reduced. The effect of temperature was probably related to a reduction in Ca²⁺ uptake. A possible change in junctional conductance produced by low temperature probably was not the reason for the decrease in rate of healing. Strontium, which replaces Ca²⁺ in the mechanical process, was also a good substitute for Ca²⁺ in healing over. The results stress the parallelism between the healing process and the contractile process in heart muscle.