Reversible motor paresis produced by local cooling of the ulnar nerve was characterized by inactivation of many motor units, by increased synchrony (coincidence of action potentials recorded simultaneously from separate points of the same muscle) and by an increase in polyphasic potentials (> 4 phases) of long duration and high amplitude. These constituted more than 60 per cent of all potentials at the height of paresis, whereas they represented 1–5 per cent of action potentials before cold block of the nerve. The duration of the simple diphasic and triphasic potentials and of the polyphasic potential remained unchanged throughout the experiment. Inactivation of motor units and synchrony of the remaining units are not only associated with, they provide an explanation for polyphasic potentials in neurogenic muscular disorders. Maximum effort is required to produce slight activity at the height of experimental paresis, and this tends to synchronize activity in different motor units. Synchrony in paresis of spinal origin is present at slight effort. Impulses synchronous at the spinal level are dispersed temporally because of varying conduction velocities along the nerve and result in slightly asynchronous action potentials from different motor units, or polyphasic potentials. Differences in conduction velocity, and, hence, temporal dispersion of action potentials, are accentuated by the local cooling used to produce experimental peripheral nerve block.