A method and apparatus for inducing fibrillation in a patient, for example to verify the efficacy of a defibrillator system. In the disclosed embodiment, an implantable cardioverter-defibrillator has an output stage coupled via a plurality of leads to the patient's heart. When it is desired to induce fibrillation in the patient, the device's control circuitry initiates an output capacitor charging cycle and subsequent delivery of a multi-phase fibrillation inducing stimulus to the heart. The multiple phases of the stimulus reflect an incremental discharging of the output capacitor. In one embodiment, the stimulus has three phases, with the first two separated by a time interval equal to or slightly less than the patient's "vulnerability window," which is defined as the time period following a paced cardiac event during which repolarization of the cardiac muscle is occurring, rendering the heart susceptible to induced fibrillation. The final phase of the multi-phase waveform reflects an untruncated discharge of the remaining energy on the output capacitor.