Rats with complete removal of the cortex anterior to bregma in adulthood (frontal cortex) were compared behaviorally and neuroanatomically to rats with similar removals at 1, 5, or 10 days of age. The age at which animals received the cortical excision made a significant difference with respect to the development of the thalamus and the remaining cortex as well as the behavioral outcome in adulthood. There was a direct relationship between cortical thickness in adulthood and the age at surgery: the earlier the lesion the thinner the cortex. Part of this anatomical effect was acute, and could be observed within 24 h of surgery, but the major reduction in thickness was not observed until adolescence. Behaviorally, the animals were administered several tests including tongue extension, grooming, beam walking, swimming, and a spatial navigation task. Like the cortical measurements, the behavioral measurements showed a clear relationship between age at surgery and behavioral outcome: the earlier the lesion in infancy, the greater the behavioral impairments. Thus, whereas rats with lesions at 10 days of age showed behavioral sparing, relative to adult operates, on every measure, rats with lesions at 5 days of age performed at about the level of adult operates on most tests and rats with lesions at 1 day had more extensive behavioral impairments than all other groups. These results imply that the effects of cortical injury in infancy are tightly correlated with the precise level of neural maturation at the time of lesion.