The male progeny of mother rats which had been undernourished during most of gestation and all of lactation were divided into two groups when weaned at 25 days. One group was nutritionally rehabilitated (G-L-) while the other was underfed for a further 9 weeks (G-L-W-) before nutritional rehabilitation. Despite this further undernutrition the G-L-W- rats eventually caught up in both body and brain weight with the G-L- group. Compared with control rats of the same age that had been well fed throughout life, the G-L- and G-L-W- animals had permanent deficits in body and regional brain weights. At 36 weeks rats were given two tests of motor co-ordination. They were required (a) to run backwards on a revolving drum and (b) to cross a chasm bridged by a ladder or parallel rods for a food reward. Both previously undernourished groups did consistently worse than controls of the same age or of similar body weight on two measures of co-ordination: falls from the revolving drum and stumbles on the bridge-crossing test. It is postulated that these differences indicate impaired cerebellar function in the prevously undernourished rats.