Methemoglobin levels were obtained before and after administration of IV lidocaine in 40 cardiac patients. Patients were given a 1-mg/kg bolus of IV lidocaine hydrochloride, started on a maintenance infusion at 2.0 mg/min, and given a second bolus of lidocaine of 0.5 mg/kg 15 minutes after the initial bolus. The maintenance infusion was adjusted from 1 to 4 mg/min according to clinical needs. Methemoglobin levels were drawn at zero, one, and six hours, and lidocaine levels were drawn at one and six hours after the initial bolus. Elevation of methemoglobin levels after lidocaine administration was statistically significant (P less than .05), but not clinically significant. The highest methemoglobin level obtained was 1.2%. Only one other patient had a level above 1%. No patient developed either signs of lidocaine toxicity or toxic levels of methemoglobin. Routine determination of methemoglobin levels is not clinically indicated following routine lidocaine administration. It may have some as-yet-undetermined value in lidocaine-toxic patients.