The repair kinetics of the gamma rays induced DNA damage was determined in murine peripheral blood leukocytes in vivo by the comet assay. Mice were exposed to 1.0 Gy of gamma rays in a 1 3 7 Cs source and samples of peripheral blood were taken from their tails at different times. The repair was evaluated per mice in separate experiments by measuring the proportion of cells with tail (comets) in each sample. An average of nearly 80% of comets was obtained at the initial time after the exposure; 2 min later the frequency decreased to 45% and continued diminishing to 22% at 15 min. This evidences the presence of a rapid repair mechanism. For a period of 25 to 40 min after exposure there was a slight but consistent increase of comets from 22 to 38% followed by a second reduction, which could be due to a late repair process that causes strand breaks and then joined them. In summary our results indicated that this system seems to be appropriate for the study of the repair capacity of cells following exposure to ionizing radiation.
