A ruthenium-oxide-type catalytic film (RuOx) was produced on carbon fiber microelectrodes by cycling the electrode potential between 0.65 and -0.85 V vs. SSCE at 100 V s(-1) in an air-equilibrated acidic solution of RuCl3. The film catalyzes oxidation of insulin in a saline buffer at pH 7.4. The minimum number of electrons transferred during the insulin oxidation at 0.65 V is 6.7. The analytical performance of the modified electrode as an amperometric detector for insulin was characterized using flow injection analysis. Linear least squares calibration curves over the range 0.10 to 1.0 mu M (five points) had slopes of 72+/-2 pA mu M-1 and correlation coefficients of 0.999 or greater. The detection limit, calculated as the concentration that would yield a signal equal to three times the root mean square noise, was 23nM and response time (t(90%)) was 40ms or less. The electrode response to 0.2 mu M insulin was stable for 3 days, The modified electrode was used for amperometric detection of exocytosis from individual pancreatic beta-cells.