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ABSTRACT. Costa S.Z.R., Cid G.C., D’Avila M.S., França T.N., Santos B.B.N., Nogueira V.A., Santos A.M. & Peixoto T.C. [Poisoning by methylene blue (methylthioninium chloride) in cat.] Intoxicação por azul de metileno (cloreto de metiltionínio) em gato. Revista Brasileira de Medicina Veterinária, 38(Supl.2):143- 146, 2016. Departamento de Epidemiologia e Saúde Pública, Universidade Federal Rural do Rio de Janeiro, Campus de Seropédica, Rodovia BR 465 Km 7, Seropédica, RJ 23851-970, Brasil. E-mail: email@example.com It’s reported a case of poisoning by methylene blue (methylthioniniumchloride) in cat that was sent to the Pathological Anatomy Sector of UFRRJ in October 2011, with history of urinary tract infection. By means of necropsy, mucous, subcutaneous tissue, medullary region of the kidney, serous bladder and adjacent tissues to the bladder with clear bluish tint, disclosure of hepatic lobular pattern, additionally bladder mucosa bluishand urine with sediments, blackened and foul odor were observed. Histopathological analysis showed micro and macrovesicular vacuolation of hepatocytes, overall in centrilobular region, which could extend to midzonal lobule region; some cells were lysed. There was, additionally, individual necrosis of rare hepatocytes and biliary retention, increase of the filtrate in tubules and glomeruli, with dilation of Bowman’s capsule, vacuolized tubular epithelium and thickening of the glomerular basement membrane. There was coagulation necrosis in some tubules of the medular area and small quantity of yellow-brown pigment in the cytoplasm of the tubular epithelium. The diagnosis of poisoning by methylene blue (methylthioninium chloride) was established based on the animal’s history and the clinical and pathological findings.