A nitrate-sensing system that consists of a micromachined sensor substrate, nitrate-permeable membrane, integrated microfluidic channels, and standard fluidic connectors has been designed, fabricated, assembled, and tested. Our microsensor was designed for in-situ monitoring of nitrate concentrations in ground water. A silver electrode was patterned for amperometric nitrate detection. An electrochemically oxidized silver electrode was used as a reference electrode. Microfluidic channels were fabricated as flow paths for the eluent and ground-water sample to the microelectrochemical (MEC) cell. The sensor also incorporates an anion-permeable membrane that is used for selective measurement of nitrate. With standard addition methods, linear calibration curves (1NlM to 1 mM) are obtained for several sensing chips. We observed that the sensor output increased only 13.9 %, even though a 100NlM nitrate sample solution contains interfering ions of the same concentration (100NlM each of PO 4-2, SO4-3, F-, Cl-).