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1. The tension developed by a muscle in response to a single supramaximal nerve volley is often the sum of two contractions, the second resulting from re-excitation of some nerve terminals during the first contraction. Re-excitation or back-firing can be prevented if the muscle nerve is stimulated twice with the second volley arriving in the muscle at a time when muscle fibres are still refractory following the first volley (Brown & Matthews, 1960). 2. Back-firing has been studied here in the medial gastrocnemius muscle of the spinal cat and responses have been recorded, either representing the summed activity of the muscle's motor supply or discharges of single functional motor axons. 3. When stimulating at a distance from the muscle, the second of two shocks is effective in suppressing back-firing over a narrower range of stimulus intervals than when stimulating close to the muscle. 4. The amount of back-firing can be reduced by stretching the muscle. At a length corresponding to the optimum for a twitch little if any back-firing remains. 5. Measurements of threshold to electrical stimulation of single functional motor axons suggests that low threshold axons are more likely to show back-firing. In the majority of cases, most or all of the motor units in the muscle have to be active before back-firing can be observed.