S-Nitrosothiols (RSNOs) are a class of molecules that function as exogenous and endogenous nitric oxide (NO) donors. RSNOs found in vivo include proteins such as S-nitrosohemoglobin and S-nitrosoalbumin, as well as low molecular weight species such as S-nitrosoglutathione (GSNO) and S-nitrosocysteine (CysNO). CAY10563 is a member of a new class of S-nitrosothiol species that act as an NO donors under acidic conditions. It decomposes with a half-life of one minute in 0.1 M phosphate buffer, pH 5.0, at 37°C and relaxes phenylephrine-constricted rat aortic strips 59% and 16% at pH 6.0 and 7.4, respectively.