The effect of γ-irradiation on the constituent components of nucleic acid was examined as the first step for studying the effect of γ-irradiation on its structure. In the present study, nucleic acid bases, nucleosides, nucleotides, and ribose, were γ-irradiated mostly in aqueous solution and in the presence of air, and the irradiation effect on those substances were examined by measuring the change in ultraviolet absorption spectrum, and the amounts of ammonia formed and ribose, phosphate, bases, and nucleosides liberated. Results obtained were as follows: 1) Ammonia was formed when solutions of bases, nucleosides, and nucleotides were ir-radiated with doses above 105R (Table 2). 2) On irradiating with doses above 106R, bases and ribose were liberated from nucleo-sides (Table 4). However, from nucleotides, corresponding nucleosides and ribose were not liberated, but bases and ribose-5-phosphate were liberated. 3) By irradiation with a dose of 2×106R, 70% of ribose was decomposed (Fig. 4). From the results mentioned above, it may be concluded that whenγ-irradiation with doses above 105R, the nucleotide is affected in several ways; deamination, scission of glyco-sidic bond between sugar and base and of ester bond between sugar and phosphate, and des-truction of base ring.