The ability of fibroblasts to perform unscheduled DNA synthesis (a measure of excision-repair) after UV irradiation was measured radioautographically for seven species at several times after several UV fluences. Both the initial rate and the maximum incorporation of [(3)H]dThd increased with the life-span of the species (shrew, mouse, rat, hamster, cow, elephant, man). Unscheduled DNA synthesis was approximately proportional to the logarithm of life-span.