Introduction: Methemoglobinemia is a rare but potentially life-threatening condition caused by oxidation of hemoglobin iron from the ferrous (Fe²) to the ferric (Fe³) state, impairing oxygen delivery and causing tissue hypoxia. Acquired forms are commonly associated with drugs and chemicals, while herbicide-induced cases are infrequently reported. Methylene blue is the first-line therapy; however, severe cases may be refractory , requiring alternative interventions. We report a case of severe propanil-induced methemoglobinemia successfully managed with red blood cell exchange transfusion. Case presentation: A previously healthy 13-year-old girl presented with vomiting, headache,tachypnea and profound cyanosis for 5 hours duration. Pulse oximetry showed persistently low oxygen saturation (~65%) despite 100% oxygen. Dark brown venous blood was noted and arterial blood gas analysis revealed paradoxically high PaO values. She denied toxin exposure; however on physical examination multiple self-inflicted injures were noted and on further questioning, she admitted deliberate ingestion of propanil, an acetanilide herbicide, 12 hours prior. She received intravenous methylene blue (cumulative 6 mg/kg) and oral ascorbic acid with adequate hydration, but clinical condition was progressively deteriorated. Serummethemoglobin remained 70%, and she developed confusion with a generalized convulsion. Due to refractory disease, urgent red blood cell exchange transfusion was performed. After three cycles, she was gradually recovered without end-organ dysfunction. Conclusion: Herbicide-induced methemoglobinemia, although rare, should be considered inpatients presenting with cyanosis unresponsive to oxygen therapy . This case demonstrates that severe methemoglobinemia may be refractory to methylene blue therapy and red blood cell exchange transfusion can be a life-saving intervention.