HPLC with electrochemical array detection (HPLC-ECD) was used to quantify 3,3′-dityrosine (diTyr) and 3-nitrotyrosine (3-NO₂-Tyr) in four regions of the human brain that are differentially affected in Alzheimer’s disease (AD). DiTyr and 3-NO₂-Tyr levels were elevated consistently in the hippocampus and neocortical regions of the AD brain and in ventricular cerebrospinal fluid (VF), reaching quantities five- to eightfold greater than mean concentrations in brain and VF of cognitively normal subjects. Uric acid, a proposed peroxynitrite scavenger, was decreased globally in the AD brain and VF. The results suggest that AD pathogenesis may involve the activation of oxidant-producing inflammatory enzyme systems, including nitric oxide synthase.