Protein sulfenic acids have been proposed as potential biochemical switches for redox signaling. This post-translational modification (PTM) is readily reversible, in contrast to some other types of oxidative PTM. Enhanced oxidative stress has been reported as a feature of hypertension, and renal function has been implicated in the development and progression of the disease in animal models such as the spontaneously hypertensive rat (SHR). However, reactive oxygen species (ROS) are also signaling molecules and may play a role in vascular function. To investigate protein sulfenation under hypertensive conditions, we examined protein extracts of SHR kidney medulla in comparison to medulla from normotensive Wistar rats. Total free thiol content of the SHR medulla was significantly lower than that of Wistar medulla, indicating enhanced oxidation of sulfhydryls. Protein sulfenation was also significantly greater in the medulla of hypertensive animals. Thioredoxin reductase activity was also reduced in SHR medulla and this may account, in part, for enhanced protein sulfenation. Purification of sulfenated proteins from SHR medulla revealed several proteins involved in processes such as metabolism, antioxidant defense, and regulation of nitric oxide synthase. Enhanced sulfenation may represent perturbed redox signaling in SHR medulla, or simply enhanced ROS generation. © 2010 American Chemical Society.