The article contains a brief review of nitric oxide (NO) metabolism assay studies in biological systems in health and in the development of pathological conditions as well as the reasons that prevent the successful development of concepts in this field. Using enzymatic sensor, the authors found that in normal core body fluids including blood contain only trace amounts of nitrite and N-nitrosocompounds (RNNO). In inflammation processes nitrite and RNNO concentrations increase in blood plasma significantly. The total concentration of nitrite and RNNO over 150 nM is obviously a proof of inflammatory process in the body. Moreover, we have shown that nitrite and RNNO concentration rise is not the result of the inducible NO-synthase activation, but is due to NO donor destruction, presumably by active oxygen species, produced by activated leukocytes.