OBJECTIVE We report a method for determining nitrite and nitrate in biological fluids by capillary electrophoresis. METHODS A Waters capillary electrophoresis system was used with a filter for detection at 214 nm. After dilution with distilled water, the sample was loaded hydrostatically onto a 60 cm x 100 microm capillary and electrophoresed at 15 kV in 15 mmol/L sulfate buffer, pH 8.0, containing 2.5% electroosmotic flow modifier. RESULTS The retention times for nitrite and nitrate were 3.9 +/- 0.8 and 4.0 +/- 0.8 min, respectively. The detection limit was 10 micromol/L for serum nitrate. The recovery was 93-115% for nitrite and 92-106% for nitrate. The within-day and between-day coefficients of variation were lower than 3.3% and 5.0%, respectively, for two pools with normal (28 micromol/L) and high (87 micromol/L) nitrate concentration. A comparison with the nitrate reductase method gave a correlation coefficient of 0.982. CONCLUSIONS Capillary electrophoresis provides many advantages, namely low cost, small sample and buffer requirements, rapidity, which makes its use particularly suitable for clinical laboratories.