The objectives of this investigation were to produce a reliable, sensitive probe to measure intracellular PO2 with a high degree of resolution and to apply this technique to biological systems. A fluorescent molecule, pyrene dissolved in paraffin oil, was encapsulated in polyacrylamide to form a probe of nanometer dimensions. The quantitative and microscopic oxygen values were determined by analyzing the quenching of the fluorescence of the probe by oxygen, as displayed on a television monitor by a silicon-intensified-target camera. The nanocapsules had a sensitivity of approximately 1 mm PO2, a spatial resolution of 0.5 micrometer, and a temporal resolution of milliseconds. Calibrated nanocapsules within nonrespiring Amoeba proteus responded to ambient partial pressures of oxygen. At two different ambient partial pressures, nanocapsules engulfed by respiring amoebas indicated an intracellular PO2 28 mm Hg less than extracellular PO2. The capsules retained their sensitivity to oxygen for at least 8 months.