Physical and chemical agents present in the environment can potentially damage mammalian DNA. Such damage is known in some cases to be repaired by the process of DNA excision repair. This process has been extensively studied utilizing the repair of ultraviolet irradiation damage as a model system. In this study we have used this system and the 5-bromodeoxyuridine photolysis assay to measure DNA excision repair in cells derived from 21 mammalian species. We have attempted to relate the DNA repair proficiencies and the average size of the repaired regions seen in the cell cultures with the various maximum life spans of the mammals studied. There was an approximate linear correlation between life span of the mammals and the number of DNA excision repair sites measured 20-22 hours following ultraviolet irradiation of the cell cultures. Several deviations from the linear relationships were observed which remain largely unexplained. The size of the repaired regions was shown not to be related to the maximum life spans of the mammals tested.