Nitrite (NO2-) and nitrate (NO3-) concentrations are usually measured as a marker of NO metabolism. The aim of this study was to validate and standardize the method for the quantification of these two metabolites (nitrite and nitrate) in serum. The samples (serum) were deproteinized with NaOH, zinc sulfate and reduced with cadmium granules. Analytical recovery of NO3- added to serum samples after reduction with cadmium granules was 96-105% (Griess reaction). Calibration curves for nitrate and nitrite were linear over the concentration range of 2.1 to 200 micromol/L in plasma. The detection limit of the assay was 0.9 micromol/L and the quantitation limit was 2.1 micromol/l. Usual values were determined from healthy subjects: the mean concentration of NO2-/NO3- (NOx) was 26.2 +/- 1.1 micromol/l (mean +/- SEM) (n = 39). Within-run precision was 6.8% and between-day precision was 2.1%.