A suitable device is needed for the direct measurement of oxygen partial pressure (P02) in blood and tissue for both clinical and research applications. Methods currently available for measuring P02 lack convenience, reliability, speed, and applicability to many situations of interest. Efforts to develop electrical sensors have not been successful. It is desirable to have a very small P02 sensor that can be inserted into a blood vessel or tissue with little disturbance, which will provide instantaneous p02 monitoring for either short or extended periods of time. A fiber-optic sensor is ideal for this application, with the advantage, for physiological use, of very small size and flexibility, safety, and low cost. A P02 sensor has been developed, based upon the principle of fluorescence quenching by oxygen. The feasibility of the sensor and its satisfactory performance have been demonstrated in in vitro and in vivo tests in preceding years. The development of the sensor as a needle probe has been accomplished. The current work has resulted in the final form of an instrument suitable for research applications.