Peroxynitrite has come into the spotlight in recent years. Its effects on proteins have been implicated in several diseases such as acute lung injury, rheumatoid arthritis, implant rejection, artherosclerosis, Parkinson's disease, and Alzheimer's disease. Peroxynitrite is thought to inactivate a variety of proteins including thiolate-ligated heme proteins such as cytochrome P450 2B1 and PGI(2) synthase, through the nitration of tyrosine residues. In previous studies it was reported that thiolate-ligated heme enzymes react with peroxynitrite to form a ferryl intermediate. In an effort to spectroscopically characterize this species in P450(BM3), we discovered that the peroxynitrite-generated intermediate is not an Fe(IV)oxo, but rather an iron-nitrosyl {FeNO}(6) complex. We present density functional calculations as well as Mossbauer and stopped-flow spectroscopic characterizations of the peroxynitrite-generated intermediate in P450(BM3).