Nitrates are the most frequently prescribed and utilized drugs worldwide. The elderly are a major population receiving nitrate therapy. Both nitrates and aging can increase in vivo reactive oxygen species (ROS) and reactive nitrogen species (RNS). To date, the effects of aging upon nitrate-induced ROS/RNS alteration are unknown. The present study tested the effects of aging upon nitrate-induced ROS/RNS alteration in vivo. 32 adults and 43 elderly unstable angina (UA) patients were subjected to 48 hours of isosorbide dinitrate intravenous injection (50 mu g/minutes) in this clinical study. Blood samples were obtained at baseline and conclusion. Outcome measures of oxidative stress included plasma malondialdehyde (MDA), myeloperoxidase (MPO), and reduced glutathione (GSH). Plasma concentrations of NOx and nitrotyrosine served as markers of RNS. Because of the significant differences in basic clinical characters between adults and the elderly, we designed an additional experiment determining ROS/RNS stress in rat cardiac tissue. Additionally, rat thoracic aortic NOS activity served as a marker indicating endothelial function. Our study demonstrated that nitrate therapy significantly increased in vivo ROS/RNS stress in the elderly compared to adult patients, confirmed by animal data. Decreased NOS activity was observed in old rats. Taken together, the present study's data suggests a synergism between nitrate treatment and the aging process.