Oxidative stress is considered a major mediator of apoptosis in several cellular systems. Peroxynitrite is a highly toxic oxidant formed by the reaction of nitric oxide with superoxide. Primary embryonic murine fibroblasts, exposed to 1 mM peroxynitrite, resulted in delayed cell death characterized by membrane blebbing, cytoplasmic shrinkage, nuclear condensation, and DNA fragmentation that were more characteristic of apoptosis than necrosis. In addition, both morphological alterations and DNA fragmentation were inhibited by the endonuclease inhibitor aurintricarboxylic acid. Pretreatment of fibroblasts with acidic fibroblast growth factor (FGF-1) markedly enhanced peroxynitrite-induced apoptosis, an observation restricted to immediate-early transcriptional and activated tyrosine phosphorylation processes. FGF-1 pretreatment had no modulatory effect on cell death elicited by other reactive oxygen species, suggesting that enhancement of apoptosis involves a unique relationship between peroxynitrite and the growth factor. Exposure of cells to peroxynitrite resulted in immediate tyrosine nitration of several polypeptides, including major targets with estimated molecular masses of 62, 68, and 77 kDa. Pretreatment with FGF-1 did not alter targets of peroxynitrite-mediated tyrosine nitration, but rather increased the total amount of this amino acid modification. Treatment with other reactive oxygen species failed to induce tyrosine nitration. Collectively, these efforts demonstrate that FGF-1 transiently renders primary fibroblasts more sensitive to peroxynitrite-induced apoptosis. In addition, results presented here predict a pivotal role for FGF-1 and peroxynitrite-induced cytotoxicity during the resolution of inflammation and repair processes in vivo.