Abstract: CHO-K1 cells were blocked at metaphase by colcemid,UV-irradaited,inchbated in medium 199 containing 3H-TαR and colcemid,and then observed by autoradiography.The authors found no convincing evidences to prove that the excision repair occurs in the DNA of metaphase chromosomes.This result is different from that of Ikushima (1977,Exp.Cell Res.,108,444).The possible causes related to such repair deficiency were suggested as follows:(1) High condensation of the DNA in metaphase chromosomes,(2) low activity of DNA polymerase β (which is known as the main repair polymerase),and (3) disintegration of nuclear envelope during metaphase.