['Obesity-related fatty liver disease is linked to mitochondrial dysfunction and oxidative stress. Nuclear factor (erythroid-derived 2)-like 2 (Nrf2) regulates mitochondrial function and is a transcriptor of multiple genes that produce antioxidants. Because Roux-en-Y gastric bypass (RYGB) improves fatty liver and decreases the oxidative stress in the liver, we hypothesized that RYGB activates Nrf2 and increases cytochrome C oxidase subunit II (COX-II) in the liver of obese rats.', 'Sprague-Dawley rats were fed a high-fat diet for 16 weeks. The obese rats underwent either RYGB (n = 20) or a sham operation (n = 20). The tissues were harvested 13 weeks postoperatively. The nuclear fraction and mitochondrial extracts were used for protein analysis with immunoblotting. Immunostaining was done on liver sections for COX-II, Nrf2, and the macrophage marker ED2 and F4/80. The gels were quantified using densitometry; P ≤ .05 was considered significant.', 'RYGB increased COX-II expression in the liver sections (3330 ± 56 versus 2056 ± 37 for RYGB versus sham, P < .001). The total (nuclear and cytoplasmic) Nrf2 expression was high in the obese sham-operated control (2456 ± 45 versus 4352 ± 76, RYGB versus sham, P < .001). However, the nuclear fraction of Nrf2 was significantly increased in the RYGB liver (2341 ± 46 versus 1352 ± 35, RYGB versus sham, P < .001). Furthermore, Nrf2 protein co-localized with the molecular markers of Kupffer cells.'] Does roux-en-Y gastric bypass improve hepatic mitochondrial function in obese rats?