Accurate and rapid measurement of the oxygen concentration in biological samples, particularly when these oxygen concentrations are low, has been the goal of many research workers. By far the most successful method has been the use of oxygen electrodes (for review see Silver, 1984). However it is limited by the stability of the electrode surface and by instabilities in the oxygen diffusion barrier (because it measures the rate of diffusion of oxygen to the cathode). An alternate method, which measures the oxygen dependent quenching of the fluorescence of pyrene butyric acid (Vaughan and Weber, 1970; Knopp and Longmuir, 1972; Opitz and Lübbers, 1984), has been much less useful because the fluorescence intensity is affected by many experimental parameters other than oxygen concentration.