A 4-month-old male indoor cat was referred for dyspnea, mental dullness and weakness, which appeared two days earlier. The cat had been adopted at 3 months of age. Physical exam showed cyanosis, dyspnea and mild hypothermia. The “spot test” was supportive of methemoglobinemia. Co-oximetry blood gas analysis revealed severe methemoglobinemia (81.40%), severe hyperchloremia and mild hyponatremia. CBC, biochemistry and urinalysis were within normal limits, blood smear showed the presence of Heinz bodies. Treatment was instituted with oxygen therapy, methylene blue 1% solution, ascorbic acid, intravenous fluid therapy. The clinical course was favorable with rapid improvement of cyanosis and methemoglobinemia (4.2%). Acquired methemoglobinemia was hypothesized. Two weeks after discharge the cat was asymptomatic but mild methemoglobinemia (15.60%) and hyperchloremia were evident. Exposure to oxidants contained in drinking water was suspected so the owners were instructed to use bottled water only. One month later the cat was asymptomatic, and methemoglobinemia and chloremia were within normal limits. Even if a congenital form due to cytochrome b5 reductase deficiency cannot be ruled out, drinking water intoxication is the most likely cause of methemoglobinemia in this cat
