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1. The kinetics of mechanical activation were examined in muscle fibres of the frog's sartorius muscle, using a voltage clamp to control membrane potential, tetrodotoxin to eliminate electrical activity and microscopic observations to determine the mechanical threshold.

2. The strength—duration curve was determined over a range of membrane potentials varying between -52 mV (rheobase) and +90 mV. At 4° C the critical duration was about 11 msec at -30 mV, 4 msec at 0 mV and 2 msec at +40 mV.

3. For pulses where V > -10 mV the threshold criterion at 4° C was that the `area above -30 mV' must exceed about 120 mV msec.

4. The effect of a brief subthreshold pulse declines with a time constant of about 3 msec at -100 mV and about 8 msec at -85 mV at 4° C.

5. Although the strength—duration curve is well fitted by assuming a first-order mechanism in which the rate of release of activator increases with membrane potential, other experiments show that the over-all mechanism is probably second order in time.

6. A short pulse must be at least 50% threshold if it is to give a visible contraction when added to a long pulse which is just below rheobase.

7. Delayed rectification was conspicuous with medium or long pulses which were just below the mechanical threshold, but short pulses could give contraction without turning on any appreciable potassium conductance.

8. The Appendix extends Falk's (1968) treatment of the charging of the tubular system under a voltage clamp.

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