Laminin is a major protein of the basement membrane (BM), a specialized extracellular matrix (ECM) of the artery wall. The potent oxidizing and nitrating agent peroxynitrous acid (ONOOH) is formed at sites of inflammation, and data implicate ONOOH in ECM damage and cardiovascular disease. Co-localization of 3-nitrotyrosine, a product of ONOOH-mediated tyrosine (Tyr) modification, and laminin has been reported in human atherosclerotic lesions. The sites and consequences of 3-nitrotyrosine (and related nitrated tryptophan) formation on laminin, ifs self-assembly and cell interactions are poorly understood. In this study murine laminin-111 was exposed to ONOOH (1-500-fold molar excess). Nitration sites were mapped and quantified using LC-MS/MS. Mono-nitration was detected at 148 sites (126 Tyr, 22 Trp), and di-nitration at 14 sites. Label-free quantification showed enhanced nitration with increasing oxidant doses. Tyr nitration was similar to 10-fold greater than at Trp. CO2 modulated damage in a site-specific manner, with most sites less extensively nitrated. 119 mononitration sites were identified with CO2 present, and no unique sites were detected. 23 di-nitration sites were detected, with 15 unique to the presence of CO2. Extensive modification was detected at sites involved in cell adhesion, protein-protein interactions and self-polymerization. Tyr-145 on the gamma 1 chain was extensively nitrated, and endothelial cells exhibited decreased adhesion to a nitrated peptide modelling this site. Modification of residues involved in self-polymerization interfered with the formation of ordered polymers as detected by scanning electron microscopy. These laminin modifications may contribute to endothelial cell dysfunction and modulate ECM structure and assembly, and thereby contribute to atherogenesis.