Implantable medical devices such as leadless cardiac pacemakers (LCP) may be configured to be delivered to a target location within the heart over a guide wire. In some cases, using a guide wire for delivery facilitates placement of devices in regions not otherwise easily reached. An LCP may include a housing and a wire lumen disposed relative to the housing. The wire lumen may be configured to allow the LCP to slide over a guide wire. In some cases, the guide wire may include a guide wire electrode that may be used to test potential implantation sites.