Voltage-gated potassium (Kv) channels regulate cell membrane potential and control a variety of cellular processes. Kv1.3 channels are expressed in several tissues and believed to participate in cell volume regulation, apoptosis, T cell activation and renal solute homeostasis. Examination of Kv1.3-deficient mice (Kv1.3(-/-)), generated by gene targeting, revealed a previously unrecognized role for Kv1.3 in body weight regulation. Indeed, Kv1.3(-/-) mice weigh significantly less than control littermates. Moreover, knockout mice are protected from diet-induced obesity and gain significantly less weight than littermate controls when placed on a high-fat diet. While food intake did not differ significantly between Kv1.3(-/-) and controls, basal metabolic rate, measured at rest by indirect calorimetry, was significantly higher in knockout animals. These data indicate that Kv1.3 channels may participate in the pathways that regulate body weight and that channel inhibition increases basal metabolic rate.