The first chapter is a summary of recent publications about the effect of HNO in the cardiovascular system, detection of HNO in vitro, and physiological reactivity of HNO. The second chapter provides synthetic details of a newly developed method of a synthesis of HNO-Mb of high purity ( 95%). The transient absorbance studies of HNO-Mb revealed completely different results than that of other myoglobin adducts such as MbO2 and NO- Mb. Formation of the Mb-Fe(III)/HNO- geminate pair was confirmed by analyses of transient absorbance spectra at 395 nm. In the third chapter rate constants of the reaction of HNO with Mb-Fe(II), Mb- Fe(III), HNO-Mb, and HNO-Mb with oxygen and nitric oxide were determined using the computational software React for Windows. In addition to the computational method, a direct method of comparison of the rates of the reaction of HNO with proteins was developed. In the last chapter synthesis of HNO adduct of hemoglobin is reported. UV-vis, EPR, and 1H NMR spectroscopic techniques were used to identify and differentiate the hydride peaks of HNO subunits of hemoglobin.