Abstract: The design and construction of a remotely deployed submersible sensor for the determination of total oxidized nitrogen (nitrate plus nitrite) in seawater is described. It was based on the flow injection principle, with solid state spectrophotometric detection of the diazotization product from the reaction of nitrite with N-(1-naphthyl)ethylenediamine dihydrochloride (N1NED) and sulfanilamide. Nitrate was pre-reduced inline with a copperized cadmium powder column. The limit of detection (3 s) was 0.1 µM nitrate (1.4 µg N/L) and the linear range was 0.1-55 µM nitrate with a 20 mm path length flow cell and a 260 µL sample loop. The linear range was modified for different environmental conditions by changing the sample loop size and(or) the flow cell path length. Results are presented for preliminary lab. studies and field trials in the Tamar Estuary and for deployments in the North Sea. The sensor was successfully deployed on three sep. occasions for complete tidal cycles (13 h) at a depth of 4 m from the bow of the RRS Challenger. Field data are compared with results from a shipboard air segmented analyzer and correlated with turbidity and salinity data.