Organic nitrites are nitric oxide (NO) donors that are used predominantly as inhalent drugs of abuse and have been shown to have immunomodulating effects. NO donors can modulate NOS activity and expression, thus altering the level of endogenous NO production. NO can react with superoxide (O-2(.-)) to form peroxynitrite (ONOO-), which can nitrate tyrosine residues in proteins and alter tyrosine phosphorylation. We investigated the effects of inhaled isobutyl nitrite (ISBN) on NOS expression, tyrosine nitration, and tyrosine phosphorylation in selected organs of rats. Following exposures of 109 and 1517 ppm ISBN for 4 h, the lung, spleen, liver, and kidney were removed and assayed by SDS-PAGE for NOS III (eNOS), NOS EI (iNOS), nitrotyrosine (NT)- and phosphotyrosine (PT)-immunoreactive proteins using specific antibodies, ISBN at 1517 ppm, but not 109 ppm, caused an increase in NOS III expression in the liver and kidney, but not in the lung and spleen, No apparent effect on NOS II expression was observed in these organs. The expressions of NT and PT protein bands (30-200 kDa) were increased in the liver and kidney, but not in the lung and spleen. This increase in NT persisted for 24 h post-exposure. Increased NOS III expression in the liver and kidney may promote peroxynitrite formation and contribute to the increase in NT and PT immunoreactivity. ISBN inhalation may thus cause changes in cellular signaling involving tyrosine phosphorylation. These findings may suggest a mechanistic basis for the apparent immunotoxicity associated with nitrite abuse.