The chemical reactions of NO are largely dictated by its redox state. Increasing evidence suggests that the various redox states of the NO group exist endogenously in biological tissues. In the case of NO+ equivalents, the mechanism of reaction often involves S-nitrosylation (transfer of the NO group to a cysteine sulfhydryl to form an RS-NO); further oxidation of critical thiols can possibly form disulfide bonds. We have physiological and chemical evidence that NMDA receptor activity can be modulated by S-nitrosylation, resulting in a decrease in channel opening. Recent data suggest that NO-, probably in the singlet (or high-energy) state, can also react with critical sulfhydryl group(s) of the NMDA receptor to down-regulate its activity; in the triplet (lower-energy) state NO- may oxidize these NMDA receptor sulfhydryl groups by formation of an intermediate such as peroxynitrite. It has also been reported that NO can react with thiol but only under specific circumstances, e.g., if an electron acceptor such as O2 is present, as well at catalytic amounts of metals like copper, and if the conditions do not favor the kinetically preferred reaction with O2.- to yield peroxynitrite. Mounting evidence in many fields suggests that S-nitrosylation can regulate the biological activity of a great variety of proteins, perhaps analogous to phosphorylation. Thus, this chemical reaction is gaining acceptance as a newly-recognized molecular switch to control protein function via reactive thiol groups such as those encountered on the NMDA receptor.