A miniaturized enzyme-modified electrode has been constructed and evaluated. The tip of a capillary-encased, carbon-fiber electrode is recessed, and tetrathiafulvalene-tetracyanoquinodimethane crystals are electrochemically deposited in the recessed tip. Flavoenzymes are placed in the recess by cross-linking with glutaraldehyde. The specific enzymes used are glucose oxidase to form a microbiosensor for glucose, and a combination of acetylcholine esterase and choline oxidase to form a microbiosensor for acetylcholine. The sensor is operated in an amperometric mode with Eapp = 150 mV versus a sodium saturated calomel electrode, and the response appears to be limited by the kinetics of the enzyme reaction. The effective maximum current density for the glucose electrode is greater than 600 microA/cm2. At low concentrations of glucose, oxygen provides a significant interference by attenuating the signal. The device is simple to prepare and has a rapid response time. Interference from ascorbate has been significantly reduced by the design and by addition of a layer of ascorbate oxidase. Although not yet suitable for use in tissue, the biosensors are suitable for detection in situations where oxygen concentrations do not frequently change.