[1-Penicillamine,2-leucine]oxytocin is a conformationally restricted analog of oxytoxin in which the half-cystine-1 and tyrosine-2 residues of the native hormone are replaced by half-penicillamine (.beta.,.beta.-dimethyl-half-cystine) and leucine, respectively. This analog is a surprisingly potent oxytocin antagonist. Extensive PMR experiments were performed to determine the conformational properties of this analog in aqueous solution, and the results were compared with the previously published model for the conformation of [1-penicillamine]oxytocin. The results are consistent with a conformation similar to that of [1-penicillamine]oxytocin except that, while [1-penicillamine]oxytocin in aqueous solution possesses two 1 .rarw. 3 (C7) type turns involving the isoleucine-3 peptide amide proton and the half-penicillamine-1 carbonyl and the asparagine-5 peptide amide proton and the isoleucine-3 carbonyl, [1-penicillamine,2-leucine]oxytocin has only the latter 1 .rarw. 3 turn. This difference between the antagonists is reflected in the different .vphi. and .psi. angles in the 3 N-terminal residues of the 2 inhibitor analogs and in differences in the preferred side-chain conformations for several residues. One particular result of these conformational differences is that, whereas for [1-penicillamine]oxytocin the tyrosine-2 side chain is unable to assume the rotamer for maximal binding to the uterine receptor, [1-penicillamine,2-leucine]oxytoxin retains conformational and dynamic properties at residues 2 and 3 which are more similar to those of oxytocin. These conformational and dynamic properties are consistent with the stronger binding and, hence, greater antagonist activity for this penicillamine analog relative to [1-penicillamine]oxytocin.