Despite the by now overwhelming evidence for the increased formation of NO • and NO • -derived oxidants in many different lung diseases, the exact contribution of NO • or its metabolites to inflammatory lung disease is still unclear. Indeed, NO • might have distinctly different roles in different stages of respiratory tract inflammatory diseases, being pro-inflammatory or pro-injurious in acute and severe stages but perhaps being protective and antiinflammatory in more stable conditions; it is uncertain whether NOS is a suitable therapeutic target in the management of inflammatory lung disease. Caution is clearly needed when interpreting observations of tyrosine nitration in animal models of disease or in human tissues, which does not automatically implicate ONOO -(as often thought), but rather indicates the formation of RNS by various mechanisms. Furthermore, animal models of chronic lung disease that usually reflect short-term or acute inflammation might not always be applicable to chronic airway diseases in humans. For instance, phagocyte degranulation, a common feature observed in association with human airway inflammatory diseases such as asthma, does not seem to occur in mouse models of asthma [43] . Therefore the importance of granule proteins, such as heme peroxidases, in the pathology of human airway diseases might not be adequately reflected in such animal models. More work with animal models more characteristic of human diseases or with biopsy materials from human subjects will be required to unravel the precise role of NO • in inflammatory lung disease, and might establish more clearly whether the pharmacological inhibition of NOS isoenzymes can be beneficial. This brings up the interesting paradox that, despite presumed adverse roles of NO • in such inflammatory lung diseases as septic shock and ARDS, NO • inhalation has been suggested as a potential therapeutic strategy to improve overall gas exchange [44] . Intriguingly, in a rat model of endotoxemia, inhalation of NO • was found to reduce neutrophilic inflammation and protein nitration [45] , again supporting the crucial involvement of inflammatory-immune cells in this protein modification.