Mitochondrial respiration by the enzymes NAD(P)H, xanthine oxidase and others results in the production of reactive oxygen species (ROS) which comprise superoxide anions (O2-.), hydroxyl radicals (OH-) and hydrogen peroxide (H2O2). These species can damage other cellular proteins and degrade nitric oxide (NO) but their action is limited by the scavenging enzyme superoxide dismutase (SOD). In pathophysiological states, such as hypertension, heart failure , obesity and diabetes ROS production is increased reflecting a state of oxidative stress (Wilcox, 2002). What is less clear is how physiological challenges to cardiovascular and renal function may impact on ROS production. This was investigated by examining how the levels enzymes involved with ROS generation in the renal cortex and medulla responded to a period of elevated dietary sodium intake. Male Wistar rats, 225-275g, were maintained on either a normal (0.3% Na) or a high salt diet (3% Na) for two weeks. Groups of 6-8 rats were placed in metabolic cages before and one and two weeks after dietary manipulation for 24h urine collections to allow estimation of fractional sodium and 8-isoprostane excretion. At the end of this period they were sacrificed, kidneys removed and separated into cortex and medulla/papilla, homogenized and proteins extracted. SOD and NAD(P)H expression and activity were measured. Data, means ▒SEM, were analysed using Student┐s `t┐ test and significance taken at P<0.05. After two weeks high sodium, fractional sodium excretion increased from 1.0▒0.2 to 21▒5 and 8-isoprostane excretion from 10▒2 to 120▒30 pg/kg/h (both P<0.001). NAD(P)H oxidase activity and protein expression increased some two-fold (both P<0.05) in the cortex, but not medulla in the rats subjected to the high salt diet. SOD activity was higher in the medulla than cortex in normal and high salt diet rats (both P<0.05). Protein expression of copper and zinc dependent SOD in cortex and medulla was similar in rats on both dietary regimes. These findings show that a short period of elevated dietary sodium intake increases NAD(P)H activity and protein levels in the cortex and is associated with elevated 8-isoprostane excretion, consistent with a rise in ROS production. Importantly, the high salt intake did not change SOD activity and therefore scavenging potential. These observations support the view that this physiological challenge of high salt intake causes a modest oxidative stress.