A description is given of two types of stationary platinum microelectrodes which electrolyze dissolved oxygen when suitable potentials are applied. With these electrodes it is possible to measure the oxygen tension in an open solution, and especially, to estimate local oxygen tensions in animal tissues. Electrodes have been constructed which can indicate oxygen tension as a function of position with a spatial resolution of 25 microns. One type of electrode, with the end of the platinum wire recessed inside the cylindrical glass tip, can be used to measure absolute oxygen tensions as often as once every five minutes. The other, an open type, with the end of the wire directly exposed to the outer medium, allows the estimation of relative oxygen tensions only, but gives continuous readings, and is useful for recording rapid changes of tension.