Abstract: An implantable medical device, such as a pacemaker, cardioverter or defibrillator, having a detachable power source, so that either the power source or the electronic circuitry can be replaced. The electronic circuitry is enclosed within a first container and the battery or power source is enclosed within a second container. The two containers are coupled by tracks slidingly connecting the containers along substantially planar sections thereof. In a first embodiment, the tracks comprise dovetail tongue and groove configurations. In a second embodiment, the tracks comprise a bayonet-type connection having male and female parts. The male part comprises two parallel arms. The female part comprises two parallel rails having a relatively inflexible section adjacent a distal end of the rail. Proximally on the rail, there is a relatively flexible section.