S-Nitrosothiols, which are possible intermediates in the pharmacological mechanisms of many vasodilators, undergo homolysis at pH 7.4 and $37^{circ}C$ to give corresponding disulfides and nitric oxide. At normal dosages, this reaction is too slow to be an obligatory step in the pharmacological mechanisms of these usually fast-acting drugs. Transfers of the nitroso moiety of S-nitrosothiols to acceptors such as thiols or certain hemeproteins are much faster. The highly unstable S-nitrosothiols such as S-nitrosocysteine and S-nitroso-N-acetyl-DL-penicillamine and the relatively stable S-nitrosothiols such as S-nitrosoglutathione and S-nitrosocaptopril had similar effects on guanylate cyclase activation as well as on the transfer of nitric oxide to hemoglobin. These findings suggest that S-nitrosothiols could act as a hypotensive agent via the transfer of nitric oxide to the heme moiety of guanylate cyclase.