Copper(II) ions catalyse the hydrolysis of glycylglycine in the pH range 3.5-6. The pH-rate profile has a maximum at pH 4.2, due to the formation of a complex CuGlyGly+. The decrease in rate at higher pH values is associated with the production of catalytically inactive CuGlyGly complexes produced by the ionization of the peptide hydrogen atom. The mechanism of the reaction is discussed and its possible biological significance considered.