A fully implantable defibrillator in which the components of the defibrillator are housed within an implantable casing. Defined within the implantable casing are two chambers. Each of the chambers is hermetically sealed from the other and from the exterior environment surrounding the implantable casing. Disposed within the first chamber is a battery and an energy storage device. A charging circuit is disposed within the second chamber and is operatively associated with the battery and the energy storage device for charging the energy storage device to a level capable of defibrillating a malfunctioning heart. A discharging circuit is disposed within the second chamber and is operatively associated with the energy storage device, for initiating the discharge of the energy storage device into the heart of the wearer. The geometry of the implantable casing is chosen to optimize the packaging of the defibrillator components therein.