1. The power spectral changes of the voluntary electromyogram (e.m.g.) and of the compound muscle action potential (c.m.a.p.) evoked by supramaximal nerve stimulation have been compared in fatigue and recovery of human voluntary muscle contraction. 2. The mean power frequency (m.p.f.) of the voluntary e.m.g. and that of the c.m.a.p. are similar during fatigue. Power spectral shift can be largely accounted for by change in shape of muscle action potentials. 3. The recovery rates of force and action potential amplitude are similar, being half complete at 30 s. M.p.f. and relaxation rate recover more slowly, being half complete at 3 min. 4. It is suggested that the recovery of power spectral changes is dependent on the metabolic state of the muscle.