Three groups of 25 pregnant rats each were injected with 25 μCi ${}^{134}{\rm Cs}$ as follows: one group was injected on day 12, another on day 14, and the third group on day 16 of gestation. A fourth group of 15 pregnant rats was injected on day 18 of pregnancy (day 0 = day plug observed). The effect of time after injection on the conceptual burden ${}^{134}{\rm Cs}$ and its distribution in the placenta, embryo or fetus, and fetal fluids was estimated by euthanizing five females daily and examining six conceptuses from each. This was done beginning with the first day and then day by day through the fifth day after the mother was injected. An exception was made with the females dosed on day 18 of pregnancy because of ensuing parturition which allowed only 3 days for such observations. The conceptus ${}^{134}{\rm Cs}$ burden was observed to be related to the size of the conceptus on the day when the mother was injected. This ${}^{134}{\rm Cs}$ burden was further observed to remain constant throughout each ...