The suitability of microcrystalline particles of lithium phthalocyanine (LiPc) as probes for electron paramagnetic resonance (EPR)-based determination of the concentration of molecular oxygen (O-2) in biological systems was previously investigated. In the present study, the interaction of nitric oxide (NO) with LiPc and the resultant characteristics of EPR line shape were investigated. The line width was linearly dependent upon the partial pressure of NO (pNO) in the range 0-100 mmHg, whereas beyond 100 mmHg, the relationship was nonlinear. A line-broadening mechanism on the basis of a dual spin model, similar to that proposed for the magnetic interaction of molecular oxygen (O-2) with LiPc, is used to interpret the linear dependence. The saturation behavior observed at higher pNO is interpreted on the basis of saturation in the surface coverage due to NO adsorption. The adsorption equilibrium was analyzed utilizing a model similar to the Langmuir adsorption isotherm. During dynamic equilibrium at ambient pressure and under 10% NO concentration, more than 85% surface coverage was apparent. The adsorption/desorption rates of NO were also determined by this model. It is concluded from this study that the line broadening of the EPR spectrum of LiPc occurs due to adsorption of NO and dipolar interaction rather than spin exchange. This behavior is similar to that of LiPc upon interaction with oxygen. Long-term exposure of LiPc to NO showed irreversible adsorption, resulting in a second broad peak. which was insensitive to further exposure to NO. However, irreversible adsorption of O-2 predominates over NO in vivo. Nitric oxide synthase (NOS) inhibitor (L-NAME) or NO donor (sodium nitroprusside) did not affect the magnitude of adsorption of O-2. These results clearly show that the EPR oximetry method that utilizes LiPc probe is not sensitive enough to determine the in vivo levels of NO (1-10 nM) in the presence of physiological concentrations of O-2.