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1. The modifications of isometric twitches after K contractures have been studied in single muscle fibres from frog muscles.

2. The twitch was greatly enhanced and somewhat prolonged after a conditioning K contracture. This potentiating effect was more evident in fibres having low values of the ratio control twitch tension/tetanus tension (P/P₀).

3. In most of the fibres, the time course of the decay of the potentiation phenomenon could be represented by the sum of three negative exponentials with average half-times of 9·1, 40 and 485 sec.

4. Between 3 and 10 min after the onset of the twitch potentiation the resting potential and action potential were unchanged.

5. Mn blocked the K contractures and prevented the onset of the twitch potentiation.

6. 40-140 sec after inducing a K contracture with 30 or 40 mM-K, a second exposure to the same [K] evoked a tension equivalent only to 57% of the first one. This partial refractoriness of the fibres is not seen when both contractures were elicited with 190 mM-K, and contrasts with the full recovery and potentiation of the twitch after a K contracture.