The tumor suppressor p53 protein stimulates nuclear base excision repair (BER) in vitro. In response to certain cellular stresses, p53 translocates to mitochondria, where it can trigger an apoptotic response. However, a potential role for p53 in modulating mitochondrial DNA repair has not yet been examined. In this study, we show that p53 also modulates mitochondrial BER. Uracil-initiated BER incorporation, which measures flux through the entire BER pathway, was lower in mitochondrial extracts from nonstressed p53 knockout mice than in wild type. The addition of recombinant p53 complemented the BER incorporation in p53 knockout extracts and stimulated BER in wt extracts. The activities of three major mitochondrial DNA glycosylases were similar in extracts from wild-type and knockout animals. Likewise, AP endonuclease activity was unaffected by the absence of p53. Gel shift experiments with recombinant p53 demonstrated that p53 did not bind to the uracil-containing substrate used in the repair assay. Polymerase gamma gap-filing activity was less efficient in p53 knockout extracts, but it was complemented with the addition of recombinant p53. Thus, we conclude that p53 may participate in mtBER by stimulating the repair synthesis incorporation step.