The female guinea-pig has been shown to represent a good model to investigate the genetic hazard of ionizing radiation in humans. The sensitivity of the guinea-pig oocytes to radiation-induced chromosome aberrations was, therefore, studied at different stages of oocyte and follicular growth. The sensitivity of oocytes enclosed in small follicles (15 weeks before ovulation) was found to be low and comparable to that of immature oocytes present at birth. The sensitivity of growing oocytes remained low and almost constant until 3 weeks before ovulation, from which time it began to increase. The most dramatic increase of sensitivity occurred during the last week preceding ovulation: about 90% of oocytes X-irradiated with 4Gy, 2 days before ovulation showed one or more chromatid interchanges, as compared to 20% for those irradiated with the same dose 1 week earlier. A comparison of our results with those found by others in the mouse shows that considerable differences of sensitivity exist between oocytes of these two species irradiated at similar stages of development. The possible reasons for these differences are discussed.