Azanone (HNO) is ar eactive nitrogen species with pronounced biological activity and high therapeutic potential for cardiovascular dysfunction. Ac ritical barrier to understanding the biology of HNO and furthering clinical development is the quantification and real-time monitoring of its delivery in living systems.H erein, we describe the design and synthesis of the first chemiluminescent probe for HNO, HNOCL-1,w hich can detect HNO generated from concentrations of Angeliss alt as lowa s1 38 nm with high selectivity based on the reaction with ap hosphine group to form as elfcleavable azaylide intermediate.W eh ave capitalized on this high sensitivity to develop ag eneralizable kinetics-based approach,whichprovides real-time quantitative measurements of HNO concentration at the picomolar level. HNOCL-1 can monitor dynamics of HNO delivery in living cells and tissues, demonstrating the versatility of this method for tracking HNO in living systems.Azanone (HNO,n itroxyl), is chemically related to nitric oxide (NO) by the addition of one electron and one proton. HNO rapidly dimerizes and eliminates to form nitrous oxide (k = 8 10 6 m À1 s À1 at 23 8 8C) [1] and reacts with molecular oxygen with estimated rate constants that range from 10 3 to 10 4 m À1 s À1 (k = 3 10 3 m À1 s À1 at 37 8 8C; [2] k = 1 10 4 m À1 s À1 at 23 8 8C; [3] k = 1.8 10 4 m À1 s À1 at 25 8 8C [4,5] ). Tw omajor biological targets responsible for HNO bioactivity are thiols,which react directly with HNO to form sulfinamides (k = 2 10 6 m À1 s À1 at 37 8 8C) [2] and the iron in heme-containing proteins. [6] While both HNO and NO can activate soluble guanylyl cyclase,only HNO acts apositive cardiac inotrope by direct reaction with cysteine residues on cardiac ryanodine receptors and the sarcoplasmic reticulum Ca 2+