Nitrosating　agents　(NAs)　stimulate　the　formation　of　powerful　mutagenic　and　carcinogenic　N-nitrosamines.　As　a　model　reaction,　the　nitrosation　of　ammonia　by　a　series　of　NAs,　i.e.,　HONO,　N2O3,　N2O4,　ONOOH,　ONCl,　ONBr,　ONSCN,　ONOCH3,　ONOC2H5,　ONNC,　ONOCl,　and　ONOSOCH3,　was　investigated　at　the　CBS-QB3　level　of　theory.　A　structure-activity　relationship　of　NAs　in　the　nitrosation　reactions　of　ammonia　was　established.　The　results　indicate　that　the　nitrosating　reactivity　of　NAs　(ON-X)　has　definite　relationship　with　the　heterolytic　bond　dissociation　energies　of　ON-X　and　H-X,　Mulliken　charges　of　N　and　X　atoms　as　well　as　number　of　the　atoms　in　the　ring　of　the　transition　state.　In　light　of　the　established　structure-reactivity　relationship　of　NAs,　ONNC　was　found　to　be　a　potential　powerful　nitrosating　species.