Tyrosine nitration is a post-translational protein modification with potentially significant biological implications. In the present study we demonstrate, for the first time, that tyrosine residues of human inducible nitric oxide synthase (NOS2) can be nitrated by peroxynitrite in vitro, leading to a decreased activity. Moreover, we show that NOS2 expressed in a skeletal muscle from septic patients is nitrated on selective tyrosine residues belonging to a canonic sequence. This phenomenon could be an endogenous mechanism of in vivo modulation of NOS2 enzymic activity.