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1. Single dissected muscle fibres were used to examine the rates of onset and reversal of the effects of zinc and tetraethylammonium (TEA) ions on the duration of the action potential and the amplitude of the twitch.

2. The increases in fall time (100-50%) of the action potential and in the amplitude of the twitch resulting from the application of zinc or TEA follow exponential time courses.

3. The time constants for the onset of the effects of 0·1 mM zinc were 11·03±0·71 sec (six fibres) and 9·24±0·77 sec (sixteen fibres) on action potential and twitch amplitude respectively. The values are not significantly different from each other.

4. The onset of the effect of zinc on action potential duration was speeded up by `detubulation' by glycerol treatment. The time constant was 3·88±0·68 sec (six fibres).

5. The time constants for the onset of the effects of 25 mM-TEA were 3·48±0·69 sec (six fibres) and 3·81±0·26 sec (nine fibres) on action potential and twitch respectively. These values are not significantly different from each other.

6. The results are consistent with the idea that zinc and TEA act as twitch potentiators by way of their effect on action potential duration. It seems likely that this effect is, in part at least, on the surface membrane, but an effect within the T-system is not ruled out.