A nitrate-sensing system that consists of a micromachined sensor substrate, anion-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 to the microelectrochemical (MEC) cell for the eluent and ground-water sample. The sensor also incorporates an anion-permeable membrane that is used for selective measurement of nitrate. With standard addition methods, linear calibration curves have been obtained and the detection limit is ~1 μM. To test selectivity, the sensor response to a 100-μM-nitrate solution and a mixture of nitrate and interfering ions (100 μM each of NO₃⁻, PO₄⁻², SO₄⁻³, F⁻, and Cl⁻) were compared. Despite the 400% increase in total ionic concentration, the sensor output only increased 13.9%.