The physical and chemical interaction of nitric oxide (NO) with stable nitroxides have been studied in both aqueous and membrane environments. The ESR spectrum of 3-carbamoyl-2,2,5,5-tetramethyl-3-pyrroline-1- yloxy (CTPO) was observed to broaden upon exposure to NO. This effect can be explained by invoking Heisenberg spin exchange as has been previously reported for molecular oxygen. No loss of total spin was observed negating the possibility of a chemical reaction between NO and CTPO. The extent of signal broadening was proportional to the concentration of NO and can thus be used to monitor NO concentration. We have used this method to observe the partitioning of NO into model membranes. We also report the use of multiquantum ESR to detect directly the effects of NO on the membrane bound spin label 12-doxylstearic acid. This methodology may prove useful for detecting NO in both aqueous and lipid environments and for examining the physical properties of NO within biological membranes.