[en] With the purpose of finding out the effects of tritium (HTO) on the embryos in their early stages of developments, five different doses of 50, 75, 100, 125 and 150 mCi of HTO were injected into the abdominal cavities of the pregnant rats on 7, 8, 9, 10 and 11 day after conception. The abnormal embryos were observed under the stereomicroscope on 18 day after conception. Anomalies were seen at the highest incidence of 81.3 % of implanted embryos and 100 % of the survived ones in the group given 100 mCi of HTO on 9 day after conception. Among them, those with cardiovascular anomalies were 68.8 % of the implanted cases and 84.6 % of the survived ones. The main heart anomalies were ventricular septal defect and abnormal branching of the great vessels. The required dose of HTO kill a half of the embryos (LD₅₀) was 132 mCi when given on 9 day after conception. The initial dose of 100 mCi of HTO was reduced to half in three days to the doses of 110, 102, 83 rad day by day. These values will be useful for comparing the teratogenic effects of HTO with those of other kinds of radiation. (author)