In a previous study, we showed that the guinea pig constitutes one of the best models to evaluate the genetic risk associated with an irradiation of the human female germ cells. Herewith, experiments were undertaken to evaluate the chromosomal radiosensitivity of oocytes of this species at two different stages of follicular development, separated by only 1 week. Female guinea pigs were X-irradiated on the ovaries, at either the beginning (day 3) or the middle (day 10) of the 17-day oestrous cycle. The doses delivered were 1 or 2 Gy. Meiotically competent oocytes were collected 1 week after irradiation (day 3) or immediately thereafter (day 10), and they were cultured to the metaphase of the first meiosis (MI) and examined for the presence of chromosome aberrations. Our data demonstrated a dramatic increase in the radiosensitivity of the oocyte during this short time interval: oocytes irradiated at the beginning of the oestrous cycle had a low frequency of chromosome aberrations, while those irradiated at the middle of the oestrous cycle (when growing Graafian follicles are clearly visible at the surface of the ovaries) exhibited heavy chromosome damage. However, we also found that oocytes irradiated at the middle of the oestrous cycle were eliminated from the ovaries in a few days, after their evolution to the MII stage. The stimulation of the first meiotic division by radiation required less than 24 h after doses of 1 or 2 Gy and was probably due to a rapid atresia of the large follicles containing the oocytes. On the basis of these results, it can be concluded that the radiosensitivity of the nearly mature guinea pig oocyte (1 week before ovulation) is clearly much higher than that of the corresponding stage in the mouse, both in terms of sensitivity to killing and to induction of chromosome aberrations.