The kinetics of DNA synthesis in the mitotic cycle of mouse corneal epithelial cells was studied after a single γ-irradiation of cells in a dose of 4 Gy at different S-phase points. Normally, corneal epitheliocyte S phase consists of S1 and S2 phases separated by an interval during which no DNA is synthesized. The duration of each phase was lengthened after single irradiation due to reparation of injuries in the cells at the expense of the time normally occupied by g1 period of the mitotic cycle. The first event during reparation is excision of damaged complex from the DNA molecule; this complex consists of labeled daughter fragment and matrix site of DNA chain that was used for the synthesis of the daughter fragment. Presumably, the entire reparation process in the cell consists of two stages: "reparative" synthesis and "additional" synthesis. The reparative synthesis, in turn, includes two stages: de novo synthesis of matrix fragment in the DNA chain at the site of the gap formation and de novo synthesis of the daughter fragment after the synthesis of the new matrix fragment is over.