Free radicals, or reactive oxygen species (ROS) and reactive nitrogen species (RNS) are highly reactive molecules which are naturally occurring by-products of normal cellular oxidation processes. In some circumstances these species may ultimately overwhelm antioxidant defenses causing damage to proteins, lipids and DNA resulting in oxidative stress (1). There are number of disease pathologies where oxidative and nitrative stress have been implicated (2). Physical activity or exercise can alter the balance of oxidative and anti-oxidative species within the human body. Although it was first assumed that due to an increased oxygen metabolism, exercise would induce an increased release of damaging reactive species, more recent research has indicated that exercise induced radical species are key to adaptive processes (3). However it remains unclear what role exercise intensity and duration play in the production of radical species and potency of free radical damage, in addition to the role of training and health status of the individual undertaking exercise. It has been proposed that damage may only occur under exhaustive exercise conditions in untrained individuals (4). In this study 3-nitrotyrosine (3-NT) in plasma was assessed as a stable marker of oxidative and nitrative stress. It is formed from the reaction between peroxynitrite (ONOO-.) and tyrosine. Healthy young adults (n=15) undertook a moderate bout of acute exercise (40 min at 70% VO2 max on a cycle ergometer). The results showed a significant increase in the level of 3-NT in response to exercise (1.07µM ± 0.49 pre- compared to 3.8µM ± 0.93 post-exercise; p=0.003) across all subjects, regardless of training status or gender. Training status did not have a significant effect on 3-NT levels post exercise, although levels in trained men (n=5) were higher than those in untrained men (n=5) (8.3µM ± 2.88 trained vs. 2.04µM ± 0.49)(p=0.08). Increased 3-NT may indicate increased presence of nitric oxide in tandem with oxidative species. In this study exercise resulted in an increase in 3-NT which was greater in trained males than untrained males. In some circumstances an increase in plasma 3-NT may cause a modification of protein function. Further investigations are required to assess the implications of raised 3-NT following exercise.