To study the changes in radiosensitivity of male and female genomes from fertilization to the pronuclear stage, the frequency of induced chromosome aberrations was examined at the first-cleavage metaphase in eggs fertilized with X-irradiated sperm, in eggs X-irradiated at the mature oocyte stage immediately before fertilization, and in fertilized eggs exposed to X-rays at various stages before DNA synthesis (1-5 h after insemination). Gametic treatment, fertilization and embryo culture were carried out in vitro. Most of the induced chromosome aberrations were chromosome-type aberrations, the frequency of chromosome fragments being the highest, followed by chromosome exchanges in both sperm and oocytes. The induction of chromosome-type aberrations was much higher in oocytes than in sperm. Chromosome-type aberrations were the main type also in fertilized eggs irradiated at the pre-DNA-synthetic stage. The radiosensitivity increased gradually with pronuclear formation (1-4h after insemination), but little difference in radiosensitivity was observed between eggs irradiated at 4 h and 5 h, corresponding to the stage when pronuclear formation was complete. The drastic change with pronuclear formation was found not only in radiosensitivity but also in the frequency of chromosome aberrations. The frequency of chromosome-type exchanges decreased drastically 2 h after insemination, and exchanges were barely observed at 5 h. The difference in radiosensitivity between male and female genomes also markedly changed with pronuclear formation, the chromosome aberration induction in the female genome being much higher than that in the male genome before accomplishment of pronuclear formation. The analysis of potentiation effects of 3-aminobenzamide and caffeine on the yield of X-ray-induced chromosome aberrations demonstrated that the increase of radiosensitivity and the decrease of chromosome-type exchange induction with pronuclear formation, may be closely correlated with alterations in chromatin configuration in the pronuclei and in repair capacity of fertilized eggs at the pre-DNA-synthetic stage. No evidence based on repair efficiency was found for the marked difference in radiosensitivity between male and female genomes during pronuclear formation.