The development of a new highly sensitive electron paramagnetic resonance probe suitable for measuring low oxygen concentrations in biological cells and tissues is reported. This probe consists of a char prepared from glucose. This char exhibits an EPR spectrum with marked oxygen-dependent linewidth broadening with a linewidth of 0.145 G under anaerobic conditions and a marked oxygen-dependent linewidth broadening of 14.5 G per percent oxygen. This marked oxygen-dependent broadening allows oxygen tensions to be measured down to mTorr values which was not previously possible with any other technique. This oximetry probe was applied to perform measurements to determine the severity of hypoxia in the ischemic rat heart. It was observed that after 10 minutes of ischemia, the oxygen concentration was 440 mTorr and it continued to fall over a 70 minute period to a value of 120 mTorr. Thus, absolute anoxia did not occur in the ischemic heart. The glucose char oximetry probe enables the measurement of oxygen in hypoxic cells and tissues with higher sensitivity than previously possible and therefore it can serve as a new important tool in characterizing the effects of low oxygen tensions in metabolic control and cellular injury.