The combined use of several nuclear magnetic resonance and restrained molecular dynamics techniques allowed the formulation of a molecular model for the preferred solution conformation of a synthetic peptide reproducing the [1-20] processing domain of the pro-ocytocin-neurophysin precursor. In the model, the conformation of the 20-membered tocin ring, with the two Cys1 and Cys6 residues bridged by a disulphide bond, is very close to that observed for isolated ocytocin in the solid state; in addition, a type II beta-turn is postulated for the 7-10 segment of the acyclic tail containing the Lys11-Arg12 processing site, and connecting ocytocin to the neurophysin domain, while the C-terminal 13-20 segment of the molecule is believed to assume a helical structure. This particular structural organization could be important in participating as the favorable conformation for optimal substrate-enzyme active site recognition and processing by specific endoproteases.