To further understand the neuroanatomical consequences following perinatal brain injury, quantitative morphometric analysis was performed on the brain of cats receiving a unilateral frontal cortical ablation between postnatal days (P) 9 and P 14 and intact control cats. In all cats, the volume of the neocortex and thalamus was measured bilaterally and that of the thalamic ventrobasal complex (VBc) was measured ipsilaterally. In addition, using stereology, the neuronal and glial (presumably) cell packing densities (CPD) and the total number of neurons and glial cells (TCN) were measured in the ipsilateral VBc. The neuronal and glial cell cross-sectional areas (CSA) were also measured in the ipsilateral VBc. The mean ipsilateral and contralateral neocortex volumes were similar between the two animal groups. There was a statistically significant 14% and 13% reduction in mean ipsilateral and contralateral thalamic volumes, respectively for the lesioned animals, while the VBc shrank by 16% relative to intact controls. The mean neuronal and glial CPD were similar between the two groups. The mean neuronal TCN was reduced by 10% in the neonatal-lesioned cats, while the mean glial TCN was reduced by 31% in the same animals, however neither value reached significance. Lastly, the mean CSA of neurons and glial cells showed a tendency to be smaller in the lesioned cats by 8% and 9%, respectively. These results: (a) indicate that the neonatal lesion caused only minor morphological brain alterations and this sharply contrast with the marked changes previously reported in cats with a similar lesion sustained prenatally; (b) suggest that the enhanced behavioral recovery and/or sparing reported for the present cats compared to fetal-lesioned animals is at least partially due to the morphological sparing reported here; (c) together with previous findings in fetal cats, support the hypothesis that the morphological changes after a neonatal neocortical lesion are qualitatively different and may depend on different mechanisms as compared to those occurring after similar damage sustained prenatally.