Milk intake of suckling rats was controlled by adjusting little size at birth to maximum (small litter, overnutrition), ten (normal litter, normal nutrition) or about 19 (large litter, undernutrition) pups. After weaning all animals received the same diet ad libitum. Overnutrition before weaning resulted in an early postnatal hyperinsulinaemia, decreased glucose tolerance and increased susceptibility to streptozotocin-induced diabetes in adulthood. However, the preweaning undernutrition seemed to have no preventive effect on diabetes susceptibility in adulthood. The study suggested that early postnatal overnutrition, which is associated with an early postnatal hyperinsulinemia, can increase diabetes susceptibility in adult life.