A quick-acting, quick-reversing method for blocking action potentials in peripheral nerves could be used in the treatment of muscle spasticity and pain. A high-frequency alternating-current (HFAC) sinusoidal waveform is one possible means for providing this type of block. HFAC was used to block peripheral motor nerve activity in an in vivo mammalian model. Frequencies from 10 to 30 kHZ at amplitudes of between 2 and 10 V were investigated. A complete and reversible motor block was obtained at all frequencies. The block threshold amplitudes showed a linear relationship with frequency, the lowest threshold being at 10 kHZ. HFAC block has three phases: an onset response; a period of asynchronous firing; and a steady state of complete or partial block. The onset response and the asynchronous firing can be minimized by using an optimal frequency-amplitude combination. In general, the onset response was lowest for the combination of 30 kHZ and 10 V.