In EPR experiments on mice it was demonstrated that a hydrophobic complex Fe2+ with diethyldithiocarbamate (DETC) is a more efficient selective NO trap than a hydrophilic complex Fe2+ with N-methyl-D-glutamine dithiocarbamate (MGD). This difference can be due to the higher stability of paramagnetic nitrosyl iron complex with DETC (MNIC-DETC) formed by NO binding to Fe2+-DETC in animal tissues in vivo. The complex analogue MNIC-MGD is reversibly oxidized in animal blood to transform into the diamagnetic EPR-silent form. The latter is detectable also in urine of animals, especially of those treated with bacterial lipopolysaccharide which initiates the enhanced NO production in the organism. We suggest that NO2 or peroxynitrite formed from endogenous NO can serve as an agent reversibly oxidizing MNIC-MGD in these animals.