Nitric oxide (NO) is generated in different cell types by the concomitant conversion of L- arginine into L-citrulline through the enzyme NO synthase. NO has been claimed to exert its action in an increasing number of physiological and pathological events. Among others, cellular communication, blood pressure regulation, homeostasis and memory formation. A huge amount of NO is also produced under pathological conditions, such as inflammation and immunological processes. This wide variety of effects is achieved through interactions of NO with some targets via a rich redox and additive chemistry. In particular, it has been shown that NO-mediated S-nitrosylation inhibits the activity of several enzymes, containing Cys residue(s) at their catalytic site, e.g. papain, caspases and cathepsin-B. Moreover, NO may modulate the activity of enzymes containing Cys residues at their regulatory regions. In this respect, NO-mediated S-nitrosylation of the regulatory Cys residues inactivates the viral-encoded aspartyl protease, a crucial enzyme for HIV-1 replication. Finally, the NO-mediated S-nitrosylation of Cys83, the single free sulfhydryl residue present in the fibronectin type-1 and epidermal growth factor-like pair of modules of the tissue-type plasminogen activator (t- PA) does not affect the catalytic (i.e. fibrinolytic) activity, but endows the serine protease with new potent vasodilatory and antiplatelet properties. In this respect, t-PA acts as a macromolecular NO-transporter. Here, the NO-mediated S-nitrosylation of some representative proteases is reviewed.