The effect of a wide variety of metal ions and metal ion complexes on the hydrolysis of uridine 2',3'-cyclic monophosphate was studied over the pH range 4.5-8.0. The greatest rate accelerations observed were 10(7)-10(8)-fold. The kinetic data obtained are interpreted in terms of a mechanism involving a rapid monodentate binding of the metal ion to the monoanionic phosphodiester and a subsequent intracomplex participation of the metal-bound hydroxo ligand, either as a nucleophile attacking the tetracoordinated phosphorus or as a base facilitating an intermolecular attack of a water molecule. No effect on the product distribution between uridine 2'- and 3'-monophosphates was observed.