Methylene blue has been advocated for multiple indications, but data reporting its use on intoxication is rare. Acquired methemoglobinemia can be caused by a wide variety of chemicals and toxins and on severe cases, with high methemoglobin levels (>20%), methylene blue is the first line treatment. We describe a case report of a 45-year-old man with acute methemoglobinemia due to involuntary exposure to n-propyl nitrate. The patient came into the emergency department conscious, with profound peripheral and central cyanosis and oxygen saturation of 88% on air, with no response to oxygen supplementation. Arterial blood gas showed methemoglobin of 22.9%. Methylene blue was administered intravenously and symptoms improved dramatically, with control arterial blood gas with 2.6% of methemoglobin. Despite the rarity of occurrence of methemoglobinemia in occupational set, and since methylene blue is highly effective, a high degree of suspicious and good training aiming for prompt diagnosis is crucial in treating this emergency situation.