1. Voltage-clamp experiments on frog slow and twitch fibres were carried out using the three-micro-electrode technique. Potassium currents were blocked by tetraethylammonium. Contraction was blocked by 2 mM-tetracaine. 2. After subtracting the linear capacitive and leakage currents, the delta V(test-control) traces from slow fibres show 'on' and 'off' charge movements similar to those observed in twitch fibres. 3. The time integrals of the 'on' and 'off' transients, Qon and Qoff, in slow fibres are, as in twitch fibres, almost equal in magnitude but opposite in direction. 4. The charge-voltage distribution is well fitted by a sigmoid curve given by (Formula: see text), which has been successfully applied to twitch fibres. Data from three fibres gave V = -25 mV, k = 13 mV, and Qmax = 7 nC/microF. Thus, intramembranous charge in slow fibres has the same steady-state voltage distribution as that in twitch fibres, but the quantity of maximum movable charge is only 1/4 to 1/3 as large. 5. Charge movement in slow fibres does not inactivate completely when the fibres are held at -20 to 0 mV for durations as long as 30 min. 6. These results show that charge movement exists in slow fibres and may serve the same function in regulating contractile activation as that postulated for twitch fibres. The lack of complete inactivation may be consistent with the ability of slow fibres to maintain maximal tension during prolonged depolarizations.