Adult female mice were injected with <tex-math>${}^{3}{\rm HOH}$</tex-math> on the day of fertilization to give prompt levels of tritium in body water of 8.5, 0.85, and 0.085 μCi/ml and started on drinking water containing 13, 1.3, and 0.13 μCi <tex-math>${}^{3}{\rm HOH}$</tex-math> per ml to maintain these levels throughout pregnancy and lactation. Their female offspring, thus exposed to tritium from conception, were sacrificed at 14 days of age, and oocytes were counted in serial sections of ovaries. Primary oocytes in these mice were decreased below control numbers by more than 90% at the 8.5 μCi/ml level and 30% at 0.85 μCi/ml. At 0.085 μCi/ml, a level substantially below those reported previously to cause measurable biological effects in mammals, a significant decrease was still observed. No evidence of a threshold was found. The data indicate that, by 14 days of age, a 50% decrease in the number of germ cells can be expected in a female mouse exposed continuously during development to approximately 2 μCi of tritium per ml body water.

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