Nitrosothiols are powerful vasodilators. They act by releasing nitric oxide, which activates the heme protein guanylate cyclase. We have studied the kinetics of nitrosothiol formation of glutathione, cysteine, N-acetylcysteine, human serum albumin, and bovine serum albumin upon reaction with nitric oxide (NO) in the presence of oxygen. These studies have been made at low pH as well as at physiological pH. At pH 7.0, contrary to published reports, nitric oxide by itself does not react with thiols to yield nitrosothiol. However, formation of nitrosothiols is observed in the presence of oxygen. For all thiols studied, the rates of nitrosothiol formation were first order in O2 concentration and second order in NO concentration and at lower concentrations (< 5 mM thiol) also depended on thiol concentrations. Analysis of the kinetic data indicated that the rate-limiting step was the reaction of NO with oxygen. Analysis of the reaction products suggest that the main nitrosating species is N2O3: RSH+N2O3-->RSNO+NO2- + H+. Rate constants for this reaction for glutathione and several other low molecular weight thiols are in the range of 3-1.5 x 10(5) M-1 s-1, and for human and bovine serum albumins 0.3 x 10(5) M-1 s-1 and 0.06 x 10(5) M-1 s-1, respectively. The data further indicate that the reaction rate of the nitrosating species N2O3 with thiols is competitive with its rate of hydrolysis. At physiological concentrations nitrosoglutathione formation represents a significant metabolic fate of N2O3, and at glutathione concentrations of 5 mM or higher almost all of N2O3 formed is consumed in nitrosation of glutathione. Implications of these results for in vivo nitrosation of thiols are discussed.