Syrian golden hamsters with removals of the medial or ventral subfields of the frontal cortex at 4 days of age were compared behaviorally and neuroanatomically with hamsters with similar removals in adulthood. The behavioral results showed that hamsters with neonatal lesions show little sparing of species-typical behaviors such as hoarding and nest building. Study of the development of animals with early lesions showed that although as young juveniles the operated hamsters did not appear to be different from their littermate controls, as they developed they failed to improve in their performance as their littermates did. As adults these early operates were thus severely impaired relative to their littermates. Nonetheless, under certain environmental conditions it was possible to show that the animals were capable of performing the behaviors nearly as proficiently as normal animals. Thus, in order to thoroughly assess the extent of behavioral sparing following early neonatal lesions, it is necessary to test animals under widely varying stimulus conditions. Finally, when the brains of neonatally operated hamsters were compared with those of animals operated on in adulthood, there were striking differences; although the area of cavity appeared smaller in the neonatal operates, their brains weighed less and the remaining neocortex was thinner.