The biological effects of S-nitrosothiols have been attributed to homolytic cleavage of the S-N bond with release of nitric oxide (NO.). Rates of NO. release from several S-nitrosothiols were determined by monitoring the oxidation of oxymyoglobin to metmyoglobin at pH 7.4; half-lives for oxymyoglobin oxidation ranged from seconds to hours. Transnitrosation reactions between S-nitrosothiols and thiol-containing amino acids, peptides, and proteins, which indicate the ability of nitrosothiols to act as nitrosyl (NO+) donors, occurred more rapidly than spontaneous NO. release. Decomposition of S-nitrosodithiols were examined as models for the reaction of nitrogen oxides with vicinal thiols on proteins. Rapid disulfide formation was accompanied by formation of hydroxylamine and nitrous oxide, indicative of nitroxyl (NO-) release. Taken together, these model studies demonstrate the ability of S-nitrosothiols to act as NO+, NO., and NO- donors under physiological conditions. Transnitrosation and acceleration of disulfide formation suggest mechanisms of regulation of protein function through the intermediacy of nitrosothiols, and support the notion that biological activities of S-nitrosothiols may be associated with heterolytic as well as homolytic mechanisms of decomposition.