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Abstract. DNA damages are the main cause of radiation-induced cell death, mutations and carcinogenesis. The study of influence of environmental conditions on the radiation damage of DNA structure is important for the wide range of medical problems. The present work is devoted to the study of alterations in DNA structure caused by γ-irradiation at the variation of counterions concentration and dose rate.The DNA solutions of the ionic strengths 0.005M and 0.15M NaCl were exposed to γ-radiation with the doses of 0-100 Gy, dose rates2.3Gy/s and 0.3 Gy/s. The lesions in DNA structure (base damages, partial denaturation) were studied by the methods of UV absorption spectroscopy and DNA melting.Radiation-induced base damages decrease at the rise of NaCl concentration in the solution and at the lowering of the dose rate. The stability of DNA secondary structure increase at the rise of ionic strength of irradiated solution. The influence of radiation dose rate on the DNA helicity reveals definitely only at the dose of 100 Gy. The relative fall of the DNA melting temperatureafter the irradiation slightly depends on the dose rate and NaCl concentration. IntroductionIonizing radiation damages all the molecules in a living cell but justinjuries in the DNA are most fatal because of the unique role of this molecule. Unrepaired damages in DNA structure cause cell death, mutations, malignant degeneration [1][2][3][4]. Radiation-induced DNA lesions are extensively studied, the most frequent types of them are single-and double strand breaks (SSB and DSB), inter-and intrastrand cross-links, destruction, modification and release of nucleobases, local breakage of hydrogen bonds (partial denaturation) [1][2][3][4].To reveal radiation-induced structural damages in DNA the model aqueous DNA solutions are widely applied in experiments [1,2]. It is possible because the secondary structure of the macromolecule in a cell and in an aqueous solution is the same and represents the B-form [5]. Besides the mechanisms of ionizing radiation action on DNA molecule in a cell and in a solution are identical: mainly it is so-called indirect action, i.e. DNA injury over its interaction with the products of water radiolysis [1,2]. The present work is devoted to the study of alterations in DNA structure caused by γ-irradiation of its aqueous solutions.