Endothelial injury or dysfunction has been proposed to be one of the initiating events of atherosclerosis and is associated with an apparent decrease in the production of the vasodilator autacoid nitric oxide (NO). The nature of the endothelial dysfunction resulting in an attenuation of NO-mediated responses is unknown although possibilities include decreased substrate availability, decreased expression of the NO synthase, imbalance between the production of endothelium-derived constricting and relaxing factors, production of an endogenous NO synthase inhibitor and overproduction of oxygen-derived free radicals. While experimental evidence has been provided to support almost all of these possibilities, increased production of superoxide anions within the vascular wall is currently favoured as an explanation for the observed changes in vascular responsiveness and the characteristic loss of the anti-adhesive properties of the endothelium in the early stages of atherosclerosis. The altered ratio of NO/superoxide anion (O<sub>2</sub> production has been proposed to alleviate intrinsic inhibition of the transcription factor NFkB and lead to enhanced expression of adhesion molecules and chemotactic factors at the endothelial surface. The aim of this short review is to summarise recent findings which suggest that an imbalance in the production of NO and O<sub>2</sub> within the vascular wall is one of the earliest events to occur in the atherogenic process.