Recently, highly oxygen-sensitive solid-state paramagnetic probes have been developed from glucose chars which are capable of providing measurements of very low, millitorr, oxygen concentrations in biological cells and tissues. With the increased interest and recognition of the presence and importance of the free-radical gas nitric oxide, NO., in biology, studies were performed aimed at characterizing the effect of NO. on this probe. It was observed that NO. exerts a similar concentration-dependent broadening as does molecular oxygen, O2. In anaerobic solutions, concentrations of NO. could be detected down to 10-100 nM levels. Measurements of NO. formation in aqueous solution were performed from a pharmacological NO. donor, which produced micromolar concentrations of NO.. This technique, however, was not sufficiently sensitive to measure physiological 1-10 nM concentrations. Thus, the glucose-char EPR probe could be used to detect pharmacological levels of NO. but was not significantly affected by the low physiological levels which occur in normal biological tissues. These results indicate that, under normal physiological conditions, the linewidth alterations measured using the glucose-char EPR probe are solely caused by oxygen.