Carbon fibres are used to construct ultramicroelectrodes 7 urn in diameter. Electrochemical operations for pre-electrolysis and measurement were examined for the sensitive determination of hydrogen peroxide. When a triangular potential ( -2 to + 2 V vs. Ag/AgCl) was applied before every measuring double pulse (first pulse, 750 mV; second pulse, 1100 mv), the limit of detection was 0.1 pM hydrogen peroxide. Reproducible determination of hydrogen peroxide is possible even in samples containing albumin. The separation of hydrogen peroxide from ascorbic acid is also possible because the oxidation potential of ascorbic acid is different from that of hydrogen peroxide. An ultramicro acetylcholine sensor was fabricated by immobilizing acetylcholinesterase and choline oxidase on the carbon fibre by entrapment with poly(viny1 alcohol)-quaternized stilbazole (PVA-SbQ). This sensor gave a linear calibration plot for the range 0.1-1.0 mM with a linear correlation coefficient of 0.9842. A rapid determination in 5 s is possible because of the thin enzyme membrane on the carbon fibre.