In experiment I the whole head of infant rats was X-irradiated (600 rad) at one of the following postnatal ages: 6, 8, 10, 12 or 16 days. Adult body weight was not affected by early treatment. Cerebellar weight was reduced by 51% in adulthood after irradiation at 6 days, but was progressively less affected by the later treatments. Forebrain weight was reduced significantly (by 7%) only after 6-day irradiation. There were permanent changes in certain brain enzyme activities (particularly cerebellar acetylcholinesterase) after 6-day irradiation. Early treatment was associated with impaired performance in adulthood in tests of motor co-ordination. These behavioural changes were particularly marked in 6-day irradiated animals who also showed the greatest alterations in cerebellar size and acetylcholinesterase activity. In experiment II 6-day-old rats were X-irradiated (600 rad) either to the whole head or forebrain (with the cerebellum shielded). Results of motor co-ordination tests in adulthood suggested that the behavioural deficits in the whole head-irradiated animals were largely a function of damage to the cerebellum.