First-order rate constants for the hydrolysis of polyuridylic acid in the absence and presence of various metal ions and their chelates have been determined under neutral and slightly acidic conditions. The hydrolysis has been shown to proceed by cleavage of non-terminal bonds rather han by stepwise release of monomeric nucleotides. Only the hydrolysis of the 3',5'-phosphodiester bonds is accelerated by metal ions, not their isomerization to 2',5'-bonds. The catalytically active species has been shown to be the monohydroxo form of the metal aquo ion. The rate accelerations are parallel, but up to 20 times greater than those obtained with uridylyl(2',5') uridine. The mechanism of the metal-ion action is discussed.