Abstract: This invention is an implantable sensor and system capable of measuring, controlling, monitoring and reporting blood constituent levels. The implantable sensor for sensing in vivo the level of at least one blood constituent in mammalian vascular tissue having at least one source of radiation from infrared through visible light, arranged to direct the radiation at the tissue where it is affected by interaction with the tissue, and at least one detector. The invention also encompasses a device for measuring and controlling the level of a blood constituent, such as glucose or oxygen, and includes an implantable infrared source module for generating an output signal representative of the sensed infrared radiation. The system includes a processor module responsive to the output signal which performs spectral analysis of the output signal and generates a control signal. The system further includes other devices for dispensing doses of medications or controlling organ function in response to the control signal.