Linear peptide H-Pro-Thr(Psi(Me,Me)pro)-Pro-OH containing a preformed cis-Pro-Thr(Psi(Me,Me)pro) tertiary amide bond cyclises instantaneously and free of formation of oligomeric structures to the cyclic tripeptide cyclo-[Pro-Thr(Psi(Me,Me)pro)-Pro]. Even at concentrations up to 10(-1) M peptide, no oligomeric structures are detected by mass spectroscopy and HPLC. 2D H-1 NMR studies of purified cyclotripeptide reveal the compound to exist in one single conformation with all peptide bonds in the cis conformation. These results indicate enhanced cyclisation tendencies of cis-amide bond containing peptides of short chain length. (C) 1999 Elsevier Science Ltd. All rights reserved.