An evaluation of the Orion selective-ion nitrate electrode for the determination of nitrate-nitrogen in waters is presented. The results obtained showed that interferences from other anions were not great at levels commonly found in waters. Recovery of added nitrate-nitrogen ranged from 83 to 105 per cent. However, nitrate-nitrogen values obtained by the electrode method on ground and surface-water samples were lower than those obtained by phenoldisulphonic acid and steam-distillation procedures. The specific-ion electrode appeared to be suited for routine analysis of well and surface waters with nitrate-nitrogen concentrations near or above the tolerance limit (10 mg l–1 of nitrate-nitrogen).