A 0.29-mm-diameter flexible electrode designed for subcutane-ous in vivo amperometric monitoring of glucose is described. The electrode was designed to allow “one-point” in vivo calibration, ie, to have zero output current at zero glucose concentration, even in thepresence of other electroreactive species of serum or blood. A valid zero point, along with a measurement of the glucose concentrationin a withdrawn sample of blood at which the current is known, defined the sensitivity in the linear response range. The electrode was four-layered, with the layers serially deposited within a 0.125-mm recess upon the tip of a polyimide-insulated 0.25-mm gold wire. The recessed structure reduced the sensitivity to movement and allowed, through control of the depth of the recess, control of the transport of glucose and thus the range of linearity. The recess contained the four polymeric layers, with a total mass less than 5 fig and no teachable components. The bottom glucose concentration-to-current transducing layer consisted of the enzyme “wiring” redox polymer poly-[(vinylimidazole) Os (bipyridine) 2Cl]+/I+, compiexed with re-combinant glucose oxidase and cross-linked with poly (ethylene glycol) diglycidyi ether, to form an electron-conducting hydrogel. The layer was overcoated with an electrically insulating layer of polyaziridine-cross-linked poly (allylamine), on which an immobilized interference-eliminating horseradish peroxidase based film was deposited. An outer biocompatible layer was formed by photo-cross-linking derivatized polyethylene oxide). The current output of a typical electrode at 10 mM glucose and at 37 C was 35 nA, the apparent Km was 20 mM, and the 10-90% response time was~ 1 min. The sensitivity varied only by±5% over a 72-h test period. The electrode tracked the blood glucose concentration in a rat model before, during, and following intraperitoneal glucose infusion. Two failure modes were observed. The first, deactivation of hydrogen peroxide-producing lactate oxidase in the interfer-ence-eliminating layer, resulted in inadequate preoxidation of interferants. Thesecond was an abrupt drop in the sensitivity of implanted electrodes,~ 7 h after implantation. The failed electrodes promptly regained their sensitivity in buffer.