The existence of RNA enzymes that catalyze phosphodiester transfer reactions suggests that RNA-catalyzed RNA replication might be possible. Indeed, it has been shown that the Tetrahymena and sunY self-splicing introns will catalyze the template-directed ligation of RNA oligonucleotides (Doudna et al., 1989, 1991). We have sought to develop a more general RNA replication system in which arbitrary template sequences could be copied by the assembly of a limited set of short oligonucleotides. Here we examine the use of tetranucleotides as substrates for a primer-extension reaction in which the 5'-nucleoside of the tetranucleotide is the leaving group, and the primer is extended by the remaining three nucleotides. When the 5'-nucleoside is guanosine, the reaction is quite efficient, but a number of competing side reactions occur, in which inappropriate guanosine residues in the primer or the template occupy the guanosine binding site of the ribozyme. We have blocked these side reactions by using the guanosine analogue 2-aminopurine riboside (2AP) as the leaving group, in a reaction catalyzed by a mutant sunY ribozyme that binds efficiently to 2AP and not to guanosine. We have begun to address the issue of the fidelity of the primer-extension reaction by measuring reaction rates with a number of different triplet/template combinations. Our results provide the basis for the further development of an RNA-catalyzed RNA replication system using short oligonucleotide substrates with novel leaving groups.