There is increasing interest in the use of magnetic stimulation as a modality for clinical examination and therapy. It is desirable that the eddy current density produced by a change in the strength of a magnetic field should be greater than the excitability threshold of the muscles or nerves to be stimulated. The eddy current waveform differs from rectangular current waveform common in electrical applications. Little basic study has been done on the effects of magnetic stimulation on threshold values, chronaxies and other aspects of the eddy current waveforms. In this paper, the authors have attempted to measure the strength-duration curve (S-D curve) of eddy currents on the sural muscle of the bullfrog and have proposed a new S-D curve formula for eddy currents in place of Weiss's equation for rectangular current waveforms.