To investigate possible permanent consequences of an early postnatal overfeeding, the following experimental model was used: Male Wistar rats were divided into three groups after birth:(1) Small litters with 3—4 newborns (over-nutrition),(2) normal litters with 12 animals (normonutrition), and (3) large litters with 20—24 newborn rats (undernutrition). After weaning all animals had free access to tap water and standard pellet diet. The serum insulin level of animals from small litters on day 15 of life was highly significantly increased as compared to the other groups. These overfed hyperin-sulinaemic rats showed a higher body weight gain during the suckling period trough juvenile life until adulthood, associated with enhanced mean food intake and resulting in an increased relative body weight (per body length) as a sign of obesity. The obesity was found to be correlated with basal hyperinsulinaemia and increased systolic blood pressure in the small-litter-adults. Moreover, the early postnatally overnourished animals developed an increased type I-like diabetes susceptibility to a “subdiabetogenic “dose of streptozotocin in adulthood. These results suggest once more that hyperinsulinism during brain differentiation, in the present experiment induced by early postnatal overnutrition, may represent a predisposing factor for the development of obesity, of increased diabetes susceptibility and also of increased cardivascular risk in later life.