DNA damage frequently leads to the production of apurinic/apyrimidinic (AP) sites, which are presumed to be repaired through the base excision pathway. For detailed analyses of this repair mechanism, a synthetic analog of an AP site, 3-hydroxy-2-hydroxymethyltetrahydrofuran (tetrahydrofuran), has been employed in a model system. Tetrahydrofuran residues are efficiently repaired in a Xenopus laevis oocyte extract in which most repair events involve ATP-dependent incorporation of no more than four nucleotides (Y. Matsumoto and D. F. Bogenhagen, Mol. Cell. Biol. 9:3750-3757, 1989; Y. Matsumoto and D. F. Bogenhagen, Mol. Cell. Biol. 11:4441-4447, 1991). Using a series of column chromatography procedures to fractionate X. laevis ovarian extracts, we developed a reconstituted system of tetrahydrofuran repair with five fractions, three of which were purified to near homogeneity: proliferating cell nuclear antigen (PCNA), AP endonuclease, and DNA polymerase delta. This PCNA-dependent system repaired natural AP sites as well as tetrahydrofuran residues. DNA polymerase beta was able to replace DNA polymerase delta only for repair of natural AP sites in a reaction that did not require PCNA. DNA polymerase alpha did not support repair of either type of AP site. This result indicates that AP sites can be repaired by two distinct pathways, the PCNA-dependent pathway and the DNA polymerase beta-dependent pathway.