[en] Eggs of Oryzias latipes were kept at 25⁰C in tritiated water from a few hours after fertilization to the day before hatching (9 days). When eggs were kept at 25⁰C in 10 μCi/ml of HTO a radioactivity of 46.8 +- 5.72 x 10^-4 μCi/ wet egg was found at an equilibrium activity level, whereas the dry egg contained only about 1% of the radioactivity of the wet egg. The radioactivity of tritium in nucleic acids (DNA plus RNA) isolated from eggs incubated with 10, 100 and 1000 μCi/ml of HTO was closely proportional to HTO concentrations. Tritium in DNA isolated from the eggs treated with 100 μCi/ml of HTO had a specific activity of 8.48 μCi/g matter. The DNA radiation dose was estimated to be 2.4 rads/day DNA. The experiment of the DNA separation into individual bases revealed that all four bases, but especially thymine, were labeled. Incorporation of tritium into both DNA and RNA was observed at the same stages at which these macromolecules were synthesized during embyogenesis. After the transfer of the tritiated eggs into tritium-free water, subsequent incubation indicated that the loss of tritium from the DNA of the eggs was very slow. In addition, the incorporation of tritium into amino acids and lipids was also studied. (auth.)