[en] The induction and rejoining of single-strand DNA breaks in Chinese hamster cells was studied by the alkaline sucros gradient technique either immediately after oxic or anoxic irradiation or after a period of incubation under oxic and anoxic conditions. A linear relationship between the dose and the yield of single-strand breaks was noted independently of the presence or absence of oxygen during radiation exposure. When quinacrine was used to inhibit rejoining of breaks during irradiation, a dose-modifying effect of oxygen by a factor of 2.7 was calculated. The energy required to induce one single-strand break was 29 eV in oxygen, and 77 eV in anoxia. Cells subjected to postirradiation incubation under oxic conditions repaired about 80% of the oxically induced breaks, and about 60% of the anoxically induced breaks within 30 minutes. Postirradiation incubaton under anoxic conditions greatly inhibited the rejoining process, and not more than 10-20% of the breaks was repaired independently whether exposure to radiation was made in the presence or absence of oxygen. (author)