The proton magnetic resonance (PMR) spectrum of acetyl-proline amide in D2O solution has been analysed by computer simulation. The spectra of the cis and the trans isomers have been separated and their PMR parameters (chemical shift and coupling constants) are given. Vicinal coupling constants of the pyrrolidine ring are interpreted by means of a Karplus zone relation. The chemical shift effect of the anisotropy of both peptide planes is considered. It follows that both isomers are puckered with Cgamma in an endo position, but the cis isomer is more rigid than the trans isomer, which moreover undergoes a small interconversion of the Cgamma and Cdelta atoms between two extreme spatial positions. The dihedral angle phi has different values in both isomers. Thus, the dihedral angle between the two peptide planes is smaller in the trans isomer than in the cis isomer.