The relationship between the strength of a shock required to defibrillate the cardiac ventricles via a bipolar catheter placed within the right ventricle and defibrillating pulse parameters has been quantitated in the dog. Specifically, when the defibrillating waveform is a truncated-exponential pulse, the threshold current, Ii, required to defibrillate depends upon the duration, d, and tilt, T, of the pulse according to the following expressions: Ii equals 1n(1 - T)-1 over T times K1 plus K2/d) and T equals Ii-If over Ii, where K1 and K2 are constants. The objective of the proposed investigation is to determine the effect of body size and heart size on the strength of shock required to defibrillate the cardiac ventricles via a bipolar catheter. Strength-duration curves for threshold, truncated exponential defibrillation shocks with various tilts will be obtained in human-sized animals.