A theoretical methodology for use in conjunction with experiment was applied to the neurohypophyseal hormone lysine vasopressin for elucidation of its accessible molecular conformations and associated flexibility, conformational transitions, and dynamics. Molecular dynamics and energy minimization techniques make possible a description of the conformational properties of a peptide in terms of the precise positions of atoms, their fluctuations in time, and the interatomic forces acting on them. Analysis of the dynamic trajectory of lysine vasopressin shows the ability of a flexible peptide hormone to undergo spontaneous conformational transitions. The excursions of an individual phenylalanine residue exemplify the dynamic flexibility and multiple conformational states available to small peptide hormones and their component residues, even within constraints imposed by a cyclic hexapeptide ring.