Abstract: Apparatus for measuring the oxygen saturation level of blood at an internal measurement site in a human or animal patient by reflectance pulse oximetry, comprising first and second light sources (5, 10), a first optical fibre (30) for transferring light from at least one said light source to the internal measurement site (35), at least one receiver (45), for receiving light from the first and second light sources, at least a second optical fibre (40) for transferring light reflected from the region of the measurement site to the receiver, and means for determining the oxygen saturation level of the blood at the internal measurement site, based on the light produced by the light sources and light received by the receiver. The optical centres of the optical fibres are separated from one another by at least 1 mm at their distal ends. The apparatus can be used in combination with a cranial axis bolt for measuring the oxygen saturation level in the brain tissue of a human or animal patient.