The kinetics and equilibria of the reaction of the thiol-containing drugs captopril ( (\mathrm{D}-3) mercapto-2-methylpropanoyl-L-proline, (\mathrm{CpSH}) ) and penicillamine ( (\beta, \beta)-dimethylcysteine, PSH) with the disulfide bonds of the neurohypophyseal peptide hormones arginine vasopres(\sin) (AVP) and oxytocin (OT) have been characterized. CpSH reacts with AVP and OT by thiol/disulfide interchange to form two peptide- (\mathrm{CpSH}) mixed disulfides, which in turn react with another molecule of (\mathrm{CpSH}) to form the reduced peptide and (\mathrm{CpSSCp}). Forward and reverse rate constants and the equilibrium constant are reported for both steps in the reaction of CpSH with AVP and OT at pH 7.00. The rate constant for the first step (\left(k_{1}\right)) is much larger than that for the second step (\left(k_{2}\right)). Also, once formed, the peptide-CpSH mixed disulfides rapidly undergo intramolecular thiol/disulfide interchange with reformation of the cyclic peptide and (\mathrm{CPSH}). PSH reacts with AVP and OT by the same two-step reaction sequence; however, the rate of the second step is very slow due to steric hindrance from the methyl groups of PSH and the PSH moiety of the peptide-PSH mixed disulfides. Using rate constants determined in this study and PSH levels in the plasma of patients on PSH therapy, it is predicted that in vivo reduction of the disulfide bonds of AVP and OT by PSH and CpSH has little effect on the plasma half-lives of AVP or OT. O 1995 Academic Press, Inc.