Among DNA repair pathways, nucleotide excision repair (NER) is able to recognize and process a wide variety of DNA lesions. The NER mechanism can be summarized in two stages: incision/excision of the lesion and DNA repair synthesis. Here, we have assessed the repair synthesis activity of protein extracts from different rat tissues by an in vitro biochemical assay that reproduces the entire NER reaction. The protein extraction procedure was adapted to rat tissues and the biochemical parameters of the assay (high salt concentration, addition of EGTA) in order to minimize non-specific nuclease activity which allows the measurement of repair activity. Using this repair assay we detected a small increase in the extent of repair synthesis in liver compared to brain and lung tissue protein extracts. Similar results were obtained using a derivative assay that allows the measurement of the incision activity of tissue protein extracts with lower incision activity in lung tissue extract.