The most recent data on mechanisms of spin trapping of nitric oxide (NO) by iron dithiocarbamate complexes in animal and plant cells and tissues are considered. The rationale is as follows: 1 In the absence of NO in cells and tissues, iron binds primarily to compounds others than dithiocarbamate ligands, e.g., tricarbonic acids. 2. Predominant binding of iron to dithiocarbamate ligands takes place only after its binding to NO, since nitrosylated iron manifests much higher affinity for these ligands that for any non-thiol compounds. 3.Within the composition of mononitrosyl dithiocarbamate complexes, iron exists predominantly in the oxidized (Fe3+) form, i.e., these complexes are originally diamagnetic. Their subsequent single-electron reduction to the paramagnetic, EPR -detectable form is mediated by endogenous or exogenous (e.g., dithionite) reducing agents. 4.Superoxide-mediated transition of paramagnetic mononitrosyl dithiocarbamate iron complexes into EPR-silent state can be accompanied by significant reduction of EPR-detectable complexes. This defect can be overcome through the use of the so-called ABC method. 5. In contrast to hydrophobic complexes fast decomposition of water-soluble mononitrosyl iron complexes in animal organisms testifies to low efficiency of these complexes in determination of NO content in animal cells and tissues.