In adult cats, paired stimulations at short intervals were applied in ventral root filaments to single motor axons innervating the peroneus tertius muscle. Paired impulses were recorded from the muscle nerve simultaneously with the electrical and mechanical responses of the muscle portion of the motor unit (muscle unit). The interstimulus interval was gradually reduced in order to determine the minimum compatible with a full activation of the muscle unit by the second impulse. 2. For motor units of all physiological types, this minimum stimulus interval was the shortest interval allowing initiation and conduction of two impulses in the axon, that is, the absolute refractory period for conduction. Its duration ranged between 0.58 and 0.88 ms, displaying no correlation with the axonal conduction velocity. 3. The amount of tension enhancement produced by paired stimulations at the shortest interval varied with the type of the motor unit: it was largest for fast-fatigable units, intermediate for fast-resistant units and smallest for slow units. 4. Paired impulses elicited by paired stimulations at the shortest possible interval arrived near the muscle at a longer interval because the second impulse was conducted at a slower velocity. The minimum interval between arrival of impulses at the muscle depended on conduction velocity and on conduction distance. 5. In motor axons to peroneus tertius, paired impulses leaving the spinal cord at a mean interval of 0.78 ms arrived near the muscle separated by a mean interval of 1.90 ms. Since such an interval always allowed full activation of the muscle unit by the second impulse, this interval is longer than the refractory period of motor units in this muscle.
