1. Pyriformis slow (and sartorius twitch) fibres from Rana temporaria were studied with a three-micro-electrode voltage-clamp technique to obtain an approximate measurement of membrane current density at a fibre end. In most experiments, a modified Ringer solution containing 2H20 and 230 mM-sucrose was used to reduce movement. 2. Linear membrane properties of slow fibres obtained with this method are consistent with results from previous studies. Measured Cm (microF/cm2) increases with fibre diameter in a manner consistent with a tubular location of part of the fibre capacitance. 3. Voltage steps to -50mV and more positive potentials result in outward membrane currents in both slow and twitch fibres. These currents develop along similar sigmoid time courses and are blocked by tetraethylammonium (TEA+) ions. The reversal potential for delayed current channels in slow fibres varies with external K+ concentration, suggesting that the delayed current in slow fibres, as in twitch, is carried by K+ ions. 4. Maximum GK,GK, in slow fibres is an order of magnitude smaller than twitch fibres. The steady-state GK-V curve of slow fibres is very broad (e-fold for approximately 15 mV), saturating at very positive voltages, whereas the GK of twitch fibres varies more steeply with voltage. 5. No evidence of inward currents was seen in slow fibres during pulses of duration up to 96 msec. 6. Slow outward currents, which do not inactivate appreciably, are seen in slow fibres during long (10 sec) pulses. Tail currents following such long pulses are very slow. The reversal potential shifts to more positive values with increasing pulse duration.