A first embodiment of the implantable real-time oximeter of the present invention is attached around a blood vessel near the site of a likely stroke to monitor large and medium size cerebral arteries. Another embodiment of the implantable real-time oximeter can be passed within cerebral blood vessels to monitor the oxygenation status of the surrounding cerebral tissues. When used within cerebral blood vessels, the emitter and detector are coplanar and contained in a small area, for example, 50-120 μm.