A single injection of tritium oxide in a dose of 1.1 MBq/g (0.5 Gy for 30 days) was shown to impair the nucleic acid metabolism in the rat spleen. The changes in the indices under study (e.g. mass, nucleic acid content and biosynthesis) increased with the dose, and the recovery started later and was incomplete. Qualitative differences were found in the effects of tritium oxide and gamma radiation with regard to the rate of DNA biosynthesis: 24 h following the injection of the radionuclide specific activity of DNA increased with dose, whereas this function was inverse in the case of gamma irradiation as it was reported in the literature.