Since a number of pathological processes such as septic shock, inflammation, graft rejection, diabetes, etc. are associated with a release of nitric oxide (NO), rapid and accurate methods of monitoring of NO concentration are of interest. Various methods for measurement of nitrite and nitrate (NO₂⁻, NO₃⁻) – the stable metabolites of NO – are commonly used for this purpose. In this paper we have shown that the proper Griess procedure for nitrite determination significantly increases the sensitivity of this method. This procedure, supplemented with deproteinization and reduction of nitrates to nitrites in the presence of NADPH-sensitive reductase, can be successfully applied for measurement of NO_x levels in human body fluids (serum, urine and CSF). Deproteinization of samples with methanol/diethylether is required and does not influence the sensitivity of detection of NO metabolites. The recovery of the method is 88%±6% (n=30). The NO_x concentrations measured by this procedure ranged from 25.0 to 39.0 μmol/l in blood, 4.6 to 14.6 μmol/l in CSF and 0.37 to 2.52 mmol/l (adjusted to creatinine concentration) in urine. The coefficient of variation for this method was between 1.3–2.2%. This method can also be recommended for measurement of NO_x produced by cells in tissue cell culture.