Purpose: To investigate the behavioural effects of prenatal irradiation on different days of gestation on the performance of two learning tasks by adult mice. Materials and methods: CD1 mice were exposed in utero to 1 Gy of 250 kV X-rays on gestational days 13, 15 or 18. Other animals were sham- exposed. Male mice were tested as adults in a radial arm maze on two learning tasks considered dependent upon either spatial memory or visual associative memory. Results: Performance of the animals on the tasks was a function of the day on which exposure occurred. Compared with sham-exposed animals, exposure on day 18 produced a highly significant deficit in performance on the spatial task, and a small improvement in the visually cued task. Exposure on day 15 produced no deficit in performance on the spatial task, but a highly significant deficit in the cued task. Exposure on day 13 produced no significant deficits on either task. Conclusions: These differential effects on performance appear to be consistent with radiation-induced insult to different memory systems within the developing mouse brain. These and further studies will help provide better estimates of the risks of radiation at different times during gestation on cognitive function in humans.