A 27-year-old man with a history of cocaine abuse ingested a large quantity of street cocaine in an apparent suicide attempt. Shortly thereafter, he developed tonic-clonic seizures and became cyanotic. An arterial blood gas sample, collected in the emergency department, appeared chocolate-brown and showed pO2 279 mmHg, pCO2 53 mmHg, and pH 7.15. Hemoglobin spectral analysis revealed significant methemoglobinemia (37%). Subsequent gas-chromatographic and mass-spectral analysis of urine confirmed the presence of cocaine (106 mg/L), benzoylecgonine (94 mg/L), and other metabolites. Further testing revealed the presence of benzocaine, a compound known to produce methemoglobinemia. A powder submitted as the "cutting" substance was shown to be benzocaine. When confronted with a possible cocaine overdose (particularly by ingestion), the physician should consider the possible clinical effects of adulterants, especially local anesthetics such as benzocaine.