In both humans and mice, fetal exposure to radiation fails to induce a persistent increase in the frequency of chromosome aberrations in blood lymphocytes. Such a low-level response to radiation exposure is counterintuitive in view of the generally accepted belief that a fetus is sensitive to radiation. To determine if this is a general phenomenon, both mammary epithelial cells and spleen cells were studied in rats. Fetuses of 17.5 days postcoitus were irradiated with 2 Gy of gamma rays, and mammary tissues were removed 6-45 weeks later. Subsequently, short-term cultures were established to detect translocations using the two-color FISH method. The results showed that translocation frequencies were not only elevated in rats irradiated as fetuses, but were also almost as high as those in rats that were irradiated as adults (12 weeks old, pregnant mothers or young virgins) and examined 6-45 weeks later. There was no evidence of higher sensitivity in fetal cells with respect to the induction of translocations. In contrast, translocation frequencies in spleen cells were not elevated in adult rats irradiated as fetuses but were increased after irradiation of adults as previously seen in mouse spleen cells and human T lymphocytes. In the case of irradiation of adult rats, the induced translocation frequencies were similar between spleen cells and mammary epithelial cells. If we take translocation frequency as a surrogate marker of potential carcinogenic effect of radiation, the current results suggest that fetal irradiation can induce persistent potential carcinogenic damage in mammary stem/progenitor cells but this does not contribute to the increased risk of cancer since it has been reported that irradiation of fetal rats of the SD strain does not increase the risk of mammary cancers. Possible reasons for this discrepancy are discussed.