A method is presented in which inorganic nitrite and nitrate are determined automatically in blood. The dialysis unit in the automated system separates nitrite and nitrate from blood protein and thus eliminates any preliminary manual de-proteinisation procedure. The diazotisation and coupling reactions involving sulphanilic acid and N-(1-naphthyl)ethylenediamine form the basis of the method. Nitrate is reduced to nitrite by a zinc column incorporated into the system, and this feature enables a rapid change from nitrite to nitrate analysis and vice versa. The method is especially suitable for the analysis of a large number of samples. Recovery from blood is reproducible and almost 100 per cent. The limit of detection is 0·1 µg per ml for nitrite, and 0·2 µg per ml for nitrate, and less than 1·0 ml of blood is needed for complete analysis. Range expansion can substantially increase the sensitivity if needed.