We have constructed and characterised in vitro a glucose sensor using glucose oxidase (GOx) incorporated onto the surface of a 200-mu m diameter carbon paste electrode containing 1,4-naphthoquinone that acted as a mediator for electron transfer. The rationale behind this design was to produce a biosensor with minimal ascorbate and oxygen interference for use in neurochemical analysis in vivo. The key to successful realisation of this aim was detailed investigation of the optimum potential for operating the electrode. An applied potential of between - 100 and - 160 mV vs. see was found to produce the best selectivity for glucose; lower values gave faradaic currents for oxygen, and higher potentials faradaic responses for ascorbate. Operating at - 160mV, the sensor had the following advantages : good sensitivity (ca. 20 mu A cm(-2) for 5 mM glucose); insensitivity to competition for reduced enzyme by oxygen; and freedom from lipid surfactant interactions. The stability of the electrode was limited, however, to a few days.