A fiber optic probe to be implanted in human body tissue for physiologic studies involving measurement and monitoring of the partial pressure of gaseous oxygen in the blood stream, which is coursing through a particular blood vessel in the body. The use of the probe is based on the principle of dye fluorescence oxygen quenching. Structurally the probe comprises two 150-micrometer strands of plastic optical fiber ending in a section of porous polymer tubing serving as a jacket or envelope for the fibers. The tubing is packed with a suitable fluorescent light-excitable dye placed on a porous adsorptive particulate polymeric support. The tubing or jacket is usually made of a hydrophobic, gas-permeable commercial material, known as Celgard, but other suitable hydrophobic gas-permeable material could be used for such structure. The fiber optic probe of the invention is of very small size and flexible so that it can easily be threaded through small blood vessels which are located in a variety of tissues of the body.