The effect of suprathreshold defibrillator shocks on cardiac function was compared in 8 isolated, perfused, contracting canine hearts using 4 durations of rectangular electrical waveforms. Defibrillation threshold was first determined for each duration; then overdose shocks of 3, 4.5, 6, and 9 times threshold current density were delivered. Left ventricular isovolumic systolic pressure decreased immediately after the shocks in proportion to the overdose shock strength. The mean defibrillation current density thresholds for the 2-, 5-, 10-, and 20-msec durations were 101, 63, 47, and 39 mA/cm2, respectively. The corresponding energy density thresholds were 4.6, 3.5, 4.4, and 6.4 mJ/cm3, respectively. The safety factor for defibrillation was defined as the current overdose ratio (delivered current density/threshold current density) required to produce a 50% decrease in isovolumic systolic pressure immediately after the shock. This dose was determined by interpolation of the curve for depression versus overdose of shock strength. The mean current safety factors for the 2-, 5-, 10-, and 20-msec durations were 3.9, 5.1, 5.4, and 5.2, respectively. The corresponding mean energy safety factors were 15, 25, 28, and 27. The margin of safety for functional depression of the heart was significantly less (p less than 0.05) for the 2 msec rectangular wave then for the 5-, 10-, and 20-msec rectangular waves.