A flow-injection enzyme electrode array system with in vivo microdialysis sampling is proposed for the simultaneous measurement of cerebral glucose, lactate, L-glutamate and hypoxanthine concentrations. The enzyme electrode array system was based on neutral red-doped silica (NRDS) nanoparticles as the electrocatalyst. These uniform NRDS nanoparticles (about 50 divided by 3 nm) were prepared by a water-in-oil microemulsion method, and characterized by the transmission electron microscopy technique. The inside neutral red dopant maintained its high electron-activity, while the outside nano silica surface prevented neutral red from leaching out into the aqueous solutions and showed high biocompatibility. These nanoparticles were then mixed with the glucose oxidase, lactate oxidase, L-glutamate oxidase or xanthine oxidase, and immobilized on the four carbon electrode array, respectively. A thin Nafion((R)) film was coated on the enzyme layer to prevent interference such as from ascorbic acid and uric acid in the dialysate. The proposed flow-injection analysis with the NRDS-enzyme electrode array system enables simultaneous monitoring of trace levels of glucose, L-glutamate, lactate and hypoxanthine in rat striatum. (C) 2004 Elsevier B.V. All rights reserved.