The Zn2+-promoted hydrolysis of the 5'-terminal ribonucleoside phosphodiester bond in chimeric ribo/deoxyribo oligonucleotide 3'-monophosphates, Up(Tp)(4) and Up(Tp),, and their dephosphorylated analogue, Up(Tp)(3)T, has been studied at various metal ion and substrate concentrations, and in the presence and absence of deoxyribooligonucleotide 3'-moriophosphates, (Tp)(n), containing no cleavable ribonucleoside phosphodiester bond, The results strongly suggest that the rate-accelerating effect of the 3'-terminal monophosphate group on the phosphodiester hydrolysis is of intramolecular origin: the Zn2+ ion bridges the favoured site of coordination, i.e. the terminal monophosphate group,and the cleaving phosphodiester bond. The 3'-monophosphate group also causes the reaction order in [Zn2+] to deviate from unity, the values obtained with Up(Tp)(3)T, Up(Tp)(4) and Up(Tp)(9) being 1.1, 1.4 and 1.7, respectively. Possibly, the intramolecular participation of the 3'-monophosphate bound Zn2+ ion is facilitated by another Zn2+ ion that stabilizes the folded conformation of the oligonucleotide chain in the reactive Zn2+/substrate macrochelate.