On-site continuous measurement of nitrate in the River Taw situated in Devon (UK) was performed automatically for two months using a recently developed ion-selective electrode (ISE). A non-submersible electrode support system was developed and a field instrument was constructed comprising the nitrate-ISEs, a reference electrode and a temperature probe connected through a pre-amplifier to a data-logger and battery supply. The nitrate-ISE was constructed using a commercially available electrode body with a membrane incorporated into the tip. This rubbery membrane contained the sensor molecule (N,N,N-triallylleucine betaine chloride, 6.5% m/m) covalently bound to polystyrene–block–polybutadiene–block–polystyrene, SBS, (43.5% m/m), with 2-nitrophenyloctyl ether (2-NPOE) as mediator (40% m/m). The Nernstian slope was −59.1 mV per decade over a linear range of 1400–0.07 mg L⁻¹ nitrate-N. The limit of detection was 0.007 mg L⁻¹ nitrate-N with a selectivity coefficient for nitrate against chloride (k^pot_{NO3−, Cl−}) of 3.4 × 10⁻³. The speed of response was less than a minute over the linear Nernstian range. The lifetime in the laboratory exceeded 5 months with no potentiometric drift over the linear Nernstian range. A temperature correction algorithm was also used to accomodate the temperature changes encountered in the river. The river nitrate results obtained with the ISEs at hourly intervals compared very favourably (R² = 0.9) with those obtained with laboratory automated chemical determinations made on contemporaneous river samples over a concentration range 0.55–2.00 mg L⁻¹ nitrate-N. The ISEs did not require re-calibration and no deterioration in performance or fouling of the membrane surface was observed during 40 days.