A series of cyclic pentapeptides was synthesized. Cyclic dipeptides, tetrapeptides and partially racemized tetra- and pentapeptides were isolated as by-products. The formation of the smaller rings is due to cleavage of the peptide chain during the cyclization reaction. NMR studies of the cyclic pentapeptides in CHFCl2 showed that several conformational changes took place when solutions prepared by dissolving crystals at -70.degree. C were gradually heated to +20.degree. C. The conformers present in the final equilibrium, usually reached below room temperature, varied for the different cyclic pentapeptides and the initial crystal conformer was hardly present. The driving force behind these conformational changes is suggested to be replacement of external H- bonds in the crystal conformer with internal H- bonds to form more stable conformers in solution. This requires conversion of cis amide bonds to trans. Attempts were made to distinguish between cis and trans amide bonds by the following NMR methods: differential solvent shifts, different band widths of the N-methyl lines, shifts induced by complexation with Eu and shift differences in 13C NMR. No conclusive results were obtained. Addition of benzene was helpful in resolving overlapping lines.