Muscle fibres from goats with myotonia congenita show characteristic responses to stimulation with intracellular currents (Adrian & Bryant, 1974). To test whether the reduced surface chloride conductance can account for these myotonic discharges, we have calculated responses of a model 'muscle fibre' to intracellular current of long duration (greater than 100 msec), assuming that the current is applied at the end of the fibre, that the fibre is of finite length, that a regenerative action potential occurs in the transverse tubular system as well as the surface, and that the potassium current in the wall of the transverse tubular system raises the potassium in the tubular lumen. In the absence of information about the kinetic parameters of the ionic currents in mammalian muscle we have used numerical values from frog muscle (Adrian, Chandler & Hodgkin, 1970). 2. In calculations with a normal surface chloride conductance a long maintained current gives only one action potential. Reduction of the chloride conductance to a half produces repetitive firing during the current; reduction to a tenth produces repetitive firing during and a small number of action potentials after the end of the current. Elimination of the tubular potassium accumulation from the calculation reduces the number but does not eliminate action potentials arising after the end of the applied current. 3. With a tenth of the normal chloride conductance calculated responses show maintained firing following a constant current if the deactivating rate of the sodium channels (betam) is reduced by 25%. As before, eliminating potassium accumulation reduces the number of post-stimulus action potentials, but it does not eliminate them altogether. 4. We conclude that in the absence of a surface chloride conductance tubular potassium accumulation could certainly contribute to the instability of the membrane, but it is clear that potassium accumulation is not the only reason for the instability of myotonic muscle fibres. The kinetics of the sodium channels are important and we do not know that they are the same in normal and myotonic fibres. Nevertheless the presence of a surface chloride conductance does stabilize the response of a fibre to constant current or to repetitive stimulation, and its absence could be a sufficient condition for myotonic behaviour.
