Rats were subjected to growth retardation either from conception to 5 days of postnatal age (FNR group) or during the period 5–25 postnatal days (IR group). The rat at 5 days of age has reached a stage of brain development comparable with the human baby at term. Hence the FNR group may serve as a model for the small-for-dates baby. Compared with well-nourished controls both restricted groups showed deficits in neurological maturation. Both brain weight and body weight in adulthood were significantly reduced by earlier growth restriction; these changes were less marked in FNR than in IR animals. Cerebellar weight was consistently reduced more than that of the remainder of the brain. Brain stem acetylcholinesterase activity in adulthood was significantly higher than normal in IR, but not in FNR animals.