A n efficient synthesis o f methionine enkephalin using a phenolic resin support is described. Analogues modified at their C-termini, such as peptide acids, amides, methyl esters and compounds formed by their reduction, were prepared conveniently from common peptide phenyl ester resins. The resin was used in the synthesis o f complex isosterically modified analogues designed to investigate the role the peptide backbone plays in receptor interaction. Free hexapeptide phenyl ester resins underwent intramolecular aminolysis liberating the corresponding cyclic peptides.