To test the hypothesis that the proliferative status of a mammalian cell determines the rate of removal of oxidative DNA damage, pre- and posthepatectomized livers in adult male Fisher 344 rats were irradiated in situ with 15.5 Gy of 137Cs-gamma-rays. At 10 and 45 min after irradiation, the livers were removed and dissociated into single cell suspensions, and the DNA damage in the isolated quiescent or proliferative liver cells was assayed by alkaline elution. Proliferative liver cells irradiated 20-24 h or 29-31 h after hepatectomy repaired their DNA damage faster than quiescent liver cells. A corresponding increase in the accessibility of the DNA to digestion by m. nuclease was observed for the post-hepatectomized liver cells. These data suggest that proliferative status is a major determinant of the rate of DNA repair in rat liver.