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Pralidoxime methylsulfate (Contrathion) is widely used to treat organophosphate poisoning. Despite animal and human studies, the usefulness of Contrathion therapy remains a matter of debate. Therapeutic dosage regimens need to be clarified and availability of a reliable method for plasma pralidoxime quantification would be helpful in this process. We here describe a high-performance liquid chromatography technique with electrochemical detection to measure pralidoxime concentrations in human serum using guanosine as an internal standard. The assay was linear between 0.25 and 50 microg mL(-1) with a quantification limit of 0.2 microg mL(-1). The analytical precision was satisfactory, with variation coefficients lower 10%. This assay was applied to the analysis of a serum from an organophosphorate poisoned patient and treated by Contrathion infusions (100 and 200 mg h(-1)) after a loading dose (400 mg).