The discovery that nitric oxide (NO) reacts with superoxide (O2.-) forming peroxynitrite (ONOO-) (1) and the proof that this reaction occurs in vivo (2,3) holds enormous implications for the understanding of free radicals in biological systems. Not only in mammalian defense mechanisms against microorganisms, but also in pathophysiology during overexposure of tissues to radicals or other highly reactive species. Peroxynitrite is a highly reactive compound with harmful effects on cells and could therefore be an important microbicidal compound. Furthermore, the reaction of superoxide with NO interferes with NO signalling mechanisms. NO is not only released in response to inflammatory agents by inflammatory cells, but is also an important messenger molecule in paracrine mechanisms and neurotransmission. Whether peroxynitrite formation is a negative side effect of NO and superoxide release, or a functional characteristic is yet to be determined, and will be discussed in this review.