[en] Rats were exposed to gamma rays or helium ions on one of days 4 to 9 of gestation. Embryonic survival was recorded as a function of intrauterine position at autopsy on day 20 of gestation. Embryos located at the ovarian and cervical ends of the uterus experienced higher rates of mortality than did their littermates located at the middle of the uterine horn. This effect was observed in litters exposed to both radiation modalities on all days studied. The influence of intrauterine position on embryonic survival was directly proportional to radiation dose and to the number of fetuses occupying the uterus horn. Under the least advantageous conditions (i.e., a crowded uterine horn exposed to a moderately high radiation dose), the cervical embryo's probability of survival was less than half that of the litter as a whole. A disproportionately high rate of embryonic mortality at the cervical position was also observed in litters irradiated under hypoxic conditions, suggesting that the non-random distribution of radiation effect was not the result of variations in oxygen concentration within the uterus. In contrast, there was no indication that intrauterine position influenced the distribution of gross morphologic abnormalities in irradiated litters