The aim of this work was to assess the capacities of some .NO-donors to release .NO, and consequently NOx in aerobic medium, or to give peroxynitrite. The method was based on the differential reactivity of serotonin (5-HT) with either NO(x) or peroxynitrite, leading in phosphate-buffered solutions to 4-nitroso- and 4-nitro-5-HT formation, respectively. Yields and formation rates of 5-HT derivatives with .NO-donor were compared to those obtained with authentic .NO or peroxynitrite in similar conditions. Aside from the capacity of diazenium diolates (SPER/NO and DEA/NO) to release .NO spontaneously, converting 5-HT exclusively to 4-nitroso-5-HT, all other .NO donors must undergo redox reactions to produce .NO. S-nitrosoglutathione (GSNO) and sodium nitroprusside (SNP) modified 5-HT only in the presence of Cu2+, GSNO yielding 6 times more 4-nitroso-5-HT than SNP. Furthermore, in the presence of Cu+, the yield of .NO-release from GSNO was 45%. The molsidomine metabolite (SIN-1), which was presumed to release both .NO and O2(7-) at pH 7.4, reacted with 5-HT differently, depending on the presence of reductant or oxidant. Under aerobic conditions, SIN-1 acted predominantly as a 5-HT oxidant and also as a poor .NO and peroxynitrite donor (15% yield of .NO-release and 14 % yield of peroxynitrite formation). The strong oxidant Cu2+, even in the presence of air oxygen, accelerated oxidation and increased .NO release from SIN-1 up to 86%. Only a small part of SIN-1 gave simultaneously .NO and O2(7-) able to link together to give peroxynitrite, but other oxidants could enhance .NO release from SIN-1.