In the scope of the study, the protective effect of hesperidin (HES), a flavanone glycoside, was investigated against sodium arsenite (NaAsO2, SA) induced heart and brain toxicity. For this purpose, 35 Sprague-Dawley male rats were divided into 5 different groups, 7 in each group. Physiological saline was given to the first group. Dose of 200 mg/kg of HES to the second group, 10 mg/kg dose of SA to the 3rd group, 100 mg/kg HES and 10 mg/kg SA to the 4th group, 200 mg/kg HES, and 10 mg/kg SA to the 5th group were given orally for 15 days. At the end of the study, biochemical, histopathological, and immunohistochemical examinations were performed on the heart and brain tissues of the rats. According to the results, SA increased malondialdehyde (MDA) and 8-hydroxy-2 '-deoxyguanosine (8-OHdG) levels and decreased glutathione (reduced, GSH) level and superoxide dismutase (SOD), catalase (CAT), and glutathione peroxidase (GPx) activities in both tissues. Also, it increased cardiac lactate dehydrogenase (LDH) and creatine kinase isoenzyme-MB (CK-MB) activities and cardiac troponin-I level (cTn-I), cerebral acetylcholine esterase activity, nuclear factor kappa-B (NF-kappa B), tumor necrosis factor-alpha (TNF-alpha), interleukin-one beta (IL-1 beta), and cysteine aspartate-specific protease-3 (caspase-3) levels. In addition, as a result of histopathological examination, it was determined that SA damaged tissue architecture, and as a result of immunohistochemical examination, it increased cardiac Bcl-2-associated X protein (Bax) and cerebral glial fibrillary acidic protein (GFAP) expression. The results have also shown that HES co-treatment has an antioxidant, anti-inflammatory, antiapoptotic effect on SA-induced toxicity and aids to protect tissue architecture by showing a regulatory effect on all values. Consequently, it was determined that HES co-treatment had a protective effect on SA-induced heart and brain toxicity in rats.