Dinitrosyl dithiolato iron complex (DNIC) has been identified as an endogenous NO carrier, yet in vivo mechanisms of NO donation remain undefined. Transnitrosylation, in which a coordinated NO group is transferred to another metal complex, has been observed in transition-metal-nitrosyl chemistry. In this study, we used three kinds of iron dithiocarbamate complexes (Fe-DTCs) as NO acceptors to elucidate in vivo transnitrosylation of diglutathionyl dinitrosyl iron complex [DNIC-(GS)(2)]. Fe-DTCs were administered to mice after the injection of DNIC-(GS)(2) and electron paramagnetic resonance (EPR) spectra were measured both in the resected organs and in the upper abdomen of living mice. The spectral feature gradually changed from an initial DNIC-(GS)(2) signal to mononitrosyl iron dithiocarbamate one, suggesting that NO-Fe-DTC was formed through in vivo reaction of DNIC-(GS)(2) with Fe-DTC. The spectral results in in vitro and in vivo systems indicate that NO-Fe-DTCs can be formed not only by the transfer of coordinated NO-group(s) in DNIC-(GS)(2) but also by the abstraction of Fe-NO group in DNIC-(GS)(2) by free DTC ligands. Transnitrosylation proceeded more rapidly in blood than in liver and kidney; and more efficiently in kidney than in liver. Further, the ability to accept NO from DNIC was dependent on water-solubility of Fe-DTCs. Thus, in vivo transnitrosylation from DNIC to exogenous iron complex could be observed and this reaction was influenced by biological constituents and properties of iron complex. These results demonstrate that the transnitrosylation from DNIC to intrinsic NO acceptors like metalloproteins has a probable significance in in vivo NO transfer process.