A comparative investigation of the induction of double-strand DNA breaks (DSBs) in the Chinese hamster V79 cells by gamma-radiation at dose rates of 1, 10 and 400 mGy/min (doses ranged from 0.36 to 4.32 Gy) was performed. The acute radiation exposure at a dose rate of 400 mGy/min resulted in the linear dose-dependent increase of the gamma-H2AX foci formation. The dose-response curve for the acute exposure was well described by a linear function y = 1.22 + 19.7x, where "y" is an average number of gamma-H2AX foci per a cell and "x" is the absorbed dose (Gy). The dose rate reduction down to 10 mGy/min lead to a decreased number of gamma-H2AX foci, as well as to a change of the dose-response relationship. Thus, the foci number up to 1.44 Gy increased and reached the "plateau" area between 1.44 and 4.32 Gy. There was only a slight increase of the gamma-H2AX foci number (up to 7) in cells after the protracted exposure (up to 72 h) to ionizing radiation at a dose rate of 1 mGy/min. Similar effects of the varying dose rates were obtained when DNA damage was assessed using the comet assay. In general, our results show that the reduction of the radiation dose rate resulted in a significant decrease of DSBs per cell per an absorbed dose.