The N-nitroso-derivative of melatonin, NOM (1-nitrosomelatonin), which has been demonstrated to be a NO• [oxidonitrogen(•)] donor in buffered solutions, is a new potential drug particularly in neurological diseases. The advantage of NOM, a very lipophilic drug, is its ability to release both melatonin and NO•, an easily diffusible free radical. In order to evaluate the distribution and the pharmacokinetics of NOM, [O-methyl-3H]NOM was administered to and followed in mice. A complementary method for monitoring NOM, EPR, was performed in vitro and ex vivo with (MGD)2–Fe2+ (iron–N-methyl-D-glucamine dithiocarbamate) complex as a spin trap. The behaviour of NOM was compared with that of GSNO (S-nitrosoglutathione), a hydrophilic NO• donor. In the first minutes following [O-methyl-3H]NOM intraperitoneal injection, the radioactivity was found in organs (6% in the liver, 1% in the kidney and 0.6% in the brain), but not in the blood. In both liver and brain, the radioactivity content decreased over time with similar kinetics reflecting the diffusion and metabolism of NOM and of its metabolites. Based on the characterization and the quantification of the EPR signal in vitro with NOM or GSNO using (MGD)2–Fe2+ complex in phosphate-buffered solutions, the detection of these nitroso compounds was realized ex vivo in mouse tissue extracts. (MGD)2–Fe2+–NO was observed in the brain of NOM-treated mice in the first 10 min following injection, revealing that NOM was able to cross the blood–brain barrier, while GSNO was not.