In neonatal Chinese hamsters, all oocytes on day 0 after birth are at pachytene and ovarian X-irradiation with 1 Gy causes no acute oocyte killing. The same treatment on day 4, however, kills all diplotene-dictyate oocytes within 48 h, and consequently only a small number of pachytene oocytes survives. We investigated late effects of X-rays on the pachytene oocytes in relation to the reproductive capacity of irradiated females. There was no difference between the 0-day irradiated females and the controls with respect to the age of sexual maturation, fertility span and the rate of age-dependent reduction in the number of oocytes due to follicular atresia. On the other hand, the 4-day irradiated females showed a marked delay in sexual maturation and a very precocious cessation of oestrous cycles. However, the degeneration of the surviving oocytes was not accelerated. Fertility tests on day 18.5 of gestation revealed that there was no difference between both irradiated groups and the controls with respect to the mean number of corpora lutea and to the occurrence of dead and abnormal conceptuses. These results indicate clearly that pachytene oocytes were not affected at all by either acute or late effects of X-rays although the dose was high enough to kill diplotene-dictyate oocytes completely.