[en] The effect of large amounts of tritiated water (HTO) from reactors on the environment is evaluated. Randomly bred mice of the Hale-Stoner-Brookhaven strain have been maintained on HTO (3 μCi/mlitre) for extended periods. First-generation animals on HTO from four weeks of age have been evaluated for changes in growth pattern, and second-generation animals (also on HTO) for breeding efficiency, dominant lethal mutation rate and bone-marrow integrity. A total of 18831 embryos were examined. Statistical analysis of these results using either Student's ''t'' test or Kruskal and Wallis rank test indicates that there was a significant (p<0.01) reduction in viable embryos and an increase in early deaths in matings involving animals drinking HTO. Beginning at eight weeks of age and monthly thereafter, the haematopoietic stem cell content of the bone marrow was determined using the exogenous spleen colony technique. Although the total cellularity of the bone marrow remains comparable in the control and treated groups, the total number of stem cells (CFU) was reduced beginning after approximately 12-20 weeks on the tritium regime. These findings indicate a reduction in the total number of pluripotent stem cells in the marrow together with the ability of this reduced number of cells to maintain normal levels of total cellularity in the bone marrow. Thus, continuous ingestion of HTO at a concentration of 3 μCi/mlitre by mice results in: (1) Reduction in number of viable embryos present in the female at late pregnancy from matings when either the female or both parents have been on HTO; (2) Increase in number of early post-implantation deaths when both parents are on HTO; (3) Reduction in bone-marrow stem cell content after 12 weeks or longer on HTO; (4) No apparent effect on breeding efficiency (percentage of females pregnant) or body weight. These results are discussed in relation to the accumulated radiation dose