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 a nitrate-permeable membrane that is used for selective measurement of nitrate. The sensor has a linear response over concentration ranging from 1 μM to 1 mM.