Nitric oxide has been used for more than 20 years as an electron paramagnetic resonance probe of oxygen binding sites in oxygen-carriers and oxygen-metabolizing metalloenzymes. The high reactivity of NO with oxygen and the superoxide anion and its high affinity for metalloproteins led biochemists to consider NO as a highly toxic compound for a living cell. This assertion has recently been reconsidered following a number of discoveries of great significance: the finding of the activation of guanylate cyclase by NO, the recognition that NO is the precursor of nitrite and nitrate ions released in the activation of macrophages by endotoxin and cytokines, evidence that NO is an Endothelium-Derived Relaxing Factor, and the discovery of NO-biosynthesis from L-arginine, a pathway common in various biological cell-to-cell signalling processes. It is now admitted that NO plays a key bioregulatory role within mammalian cells, between cells of different types and in the host defence response. In the present review we have attempted to give a general picture of what is known of the chemical, physical, biochemical and biophysical properties of NO among the various nitrogen oxides. We have focussed on the structural information that can be obtained by electron paramagnetic resonance spectroscopy of nitrosyl-metalloprotein complexes. Finally we have shown how molecular targets of nitric oxide can be characterized, within whole cells, by electron paramagnetic resonance spectroscopy.