In this paper a technique for predicting thoracic impedance to defibrillator pulses is described. The impedance to low-current (1.0 mA) high-frequency (10-500kHz) sinusoidal current is used as an indicator of the impedance of the thorax to high-current, damped sinusoidal waveform pulses. Results from 71 dogs to which defibrillator shocks of 4 to 220 A peak current were applied show that thoracic impedance can be predicted by this method. This information indicates that it is possible to design a defibrillator that can automatically measure chest impedance prior to a defibrillation shock and deliver a predetermined peak current to the subject.