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1. An RC circuit employing a piezo-electric crystal was used to differentiate the tension output of electrically stimulated frog toe muscle.

2. The rate of tension development curves (dP/dt) usually displayed an initial step-wise increase in the rate of tension development, and often showed further discernible steps in tension acceleration. The successive stages of tension acceleration tended to be equal in duration, and approximated the length of the latent period (ca. 4 msec at room temperature). These observations suggest a synchronous turnover of the links between the myofilaments during the initial interval following stimulation, with an over-all cycle time at room temperature of about 8 msec.

3. Lowering the temperature produced proportionate alterations in the duration as well as the magnitude of each successive phase of tension development, with a Q₁₀ of approximately 2.

4. Characteristic changes in the pattern of tension generation were seen with alterations in muscle length, and at lengths greater than 120% of the standard length the irregularities in the dP/dt curves disappeared and the rate of tension development increased in linear fashion. This behaviour could be accounted for by the hypothetical S-filaments connecting the free ends of the thin actin filaments across the H-zone.

5. The effects of nine twitch potentiators were studied. Only perchlorate produced prominent increases in the earliest phase of tension development and in maximum tetanus tension—properties expected with an increase in the excitation-induced release of calcium ion into the sarcoplasm (increased intensity of the `active state').

6. The time of onset of the increases in tension acceleration which were produced by twitch potentiators did not correlate with their capacity to lower the `mechanical threshold', indicating that the amount of calcium released by the action potential is not necessarily altered by a shift in the membrane potential at which calcium release occurs.

7. All potentiators studied proved capable of (a) augmenting the rate of increase of tension generation (slope of dP/dt curves) during their rapid phase, as well as (b) increasing the interval during which the dP/dt curves for twitch and tetanus coincide. These effects occurred separately. It is suggested that both of these effects may be explained on the basis of delayed removal of sarcoplasmic calcium ion, resulting in prolongation of the `active state'.