A new type of chemically modified electrode (CME) was fabricated by electrodeposition of palladium(II)-substituted Dawson type heptadecatungstodiphosphate, K-8[P2W17O61Pd(H2O)] (abbreviated as P2W17Pd in the following), onto a carbon fiber microelectrode (CFME). A pair of waves was observed on the P2W17Pd CFME, which is ascribed to the redox process of the palladium center in the heteropolytungstate. After continuous potential scanning for 30 min in a pH 4.0 buffer, 92% of the original electrode response remained for the P2W17Pd CFME. The P2W17Pd CFME had high electrocatalytic activity for nitrite reduction and exhibited good reproducibility and stability. The catalytic peak current was found to be linear with the nitrite concentration in the range of 1.0 x 10(-7) similar to 1.2 x 10(-3) mol l(-1) (at 25 degrees C) with a correlation coefficient of 0.9886 The detection limit (signal/noise = 3) was found to be 2.0 x 10(-8) mol l(-1). The response time of the microsensor for nitrite measurement was less than 15 s. For 10 parallel measurements of 1.0 x 10(-5) mol l(-1) nitrite, the relative standard deviation (RSD) was found to be 4.5%. The sensitivity of the microsensor was 0.57 nA (mu mol l(-1))(-1). The P2W17Pd CME was applied successfully as an electrochemical detector (ECD) to determine the nitrite level in rat brain by flow injection analysis (FIA) coupled with microdialysis sampling. The linear range was over three orders of magnitude and the detection limit was 3.0 pmol for nitrite determination. The mechanism of the catalytic reaction was also addressed.