Single frog skeletal muscle fibres bathed in a relaxing solution were cut close to the tendon and mounted across a single Vaseline gap so that a short segment of intact terminated fibre extended beyond one side of the gap. 2. A compensating circuit, set with a micro-electrode in the terminated fibre segment, was used both to correct total current for external current crossing the gap and to correct pool voltage for the voltage drop across the fibre segment in the gap. 3. The micro-electrode was then removed and the fibre voltage-clamped using the compensating circuit. This allowed movement without damage under controlled voltage. 4. Strength-duration curves for contraction thresholds of cut fibres exposed externally to TTX Ringer solution and internally to a predominantly K glutamate solution were similar to strength-duration curves reported for intact fibres. 5. The change from TTX Ringer to a predominantly (TEA)2SO4 external solution shifted the strength-duration curve for cut fibre contraction thresholds in the negative direction as reported for intact fibres. 6. When studied at 3-4 degrees C, fibres from warm-adapted frogs appeared to have higher contraction thresholds than fibres from cold-adapted frogs. 7. Delayed rectifier currents recorded from cut fibres were similar to those reported for intact fibres.
