A novel method of eliminating interferences which arise from electrochemically active substances is described. The sample solutions are passed through a platinum tube electrode inserted into the sample flow line, where they are subjected to DC pre-electrolysis before delivery to a detector cell containing an enzyme electrode. This method was specifically developed to avoid the interference of monoamines and ascorbic acid when brain extracellular fluid is analysed for glutamate, glucose or lactate using an amperometric enzyme flow-cell. HPLC analysis of the reaction products showed that the pre-oxidation treatment efficiently removes the principal interferents, but leaves unaffected the analyte of interest.