NADPH oxidase 4 (NOX4) is the most abundant NOX isoform in the kidney; however, its importance for renal function has only recently emerged. The NOX4-dependent pathway regulates many factors essential for proper sodium handling in the distal nephron. However, the functional significance of this pathway in the control of sodium reabsorption in the initiation of chronic kidney disease is not established. We show that genetic ablation of Nox4 in Dahl salt-sensitive (SS) rat attenuates a high-salt (HS)-induced increase in epithelial Na+ channel (ENaC) activity in the cortical collecting duct. We also found that H 2 O 2 upregulated ENaC activity, and H 2 O 2 production was reduced in both the renal cortex and medulla in SS Nox4−/− rats fed an HS diet. NaCl cotransporter expression was increased in the streptozotocin model of hyperglycemia-induced renal injury compared to healthy controls, while expression values between SS and SS Nox4−/− groups were similar. ENaC activity in hyperglycemic animals was elevated in SS but not SS Nox4−/− rats. These data emphasize a critical contribution of the NOX4-mediated pathway in maladaptive upregulation of ENaC-mediated sodium reabsorption in distal nephron in the conditions of HS-and hyperglycemia-induced kidney injury.