At the gastrula phase of development, just after the onset of implantation, the embryo proper is characterized by extremely rapid cell proliferation. The importance of DNA repair is illustrated by embryonic lethality at this stage after ablation of the genes involved. Insight into mutation induction is called for by the fact that women often do not realize they are pregnant, shortly after implantation, a circumstance which may have important consequences when women are subjected to medical imaging using ionizing radiation. We screened gastrula embryos for DNA synthesis, nuclear morphology, growth, and chromosome aberrations (CA) shortly after irradiation with doses up to 2.5 Gy. In order to obtain an insight into the importance of DNA repair for CA induction, we included mutants for the non-homologous end joining (NHEJ) and homologous recombination repair (HRR) pathways, as well as Parp1-/- and p53+/− embryos. With the pUR288 shuttle vector assay, we determined the radiation sensitivity for point mutations and small deletions detected in young adults. We found increased numbers of abnormal nuclei 5 h after irradiation; an indication of disturbed development was also observed around this time. Chromosome aberrations 7 h after irradiation arose in all genotypes and were mainly of the chromatid type, in agreement with a cell cycle dominated by S-phase. Increased frequencies of CA were found for NHEJ and HR mutants. Gastrula embryos are unusual in that they are low in exchange induction, even after compromised HR. Gastrula embryos were radiation sensitive in the pUR288 shuttle vector assay, giving the highest mutation induction ever reported for this genetic toxicology model. On theoretical grounds, a delayed radiation response must be involved. The compromised developmental profile after doses up to 2.5 Gy likely is caused by both apoptosis and later cell death due to large deletions. Our data indicate a distinct radiation-sensitive profile of gastrula embryos, including some stage-specific aspects that are not as yet understood.