The present application describes an intravascular implantable pacing and/or defibrillation system. The described system includes a pulse generator that is implantable within a blood vessel and proportioned to blood flow through the blood vessel, and at least one lead attachable to the pulse generator. During implantation, the pulse generator is introduced into a patient's vasculature, advanced to a desired vessel and anchored in place within the vessel. The lead or leads are placed within the heart or surrounding vessels as needed to deliver electrical pulses to the appropriate location.