Postnatal development of dendritic spines (DS) on the 500-microns-proximal region of the apical dendrite of large layer V pyramidal neurons of the somatosensory cortex (Par 1) was studied with the rapid Golgi method in micrencephalic rats produced after exposure to 100 cGy X-irradiation at embryonic day 18. Treated rats examined at the age of 15 days had more DS than age-matched controls, whereas the reverse occurred in rats aged 30 days. After this time the number of DS decreased in normal rats. As a result, irradiated and control rats aged 90 days had about the same number of DS in the proximal region of the apical dendrite. These results suggest that development of DS appears early, and that natural overproduction and elimination of DS is impaired in the cerebral cortex of micrencephalic rats.