The distribution of cells expressing calcium-permeable α-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid receptors (CP-AMPARs) in the somatosensory cortex of rats at different developmental stages was studied using a kainate-stimulated Co2+-labeling assay in a quantitative manner. The applicability of this assay for identifying CP-AMPAR-expressing cells was first verified using cultured rat cortical neurons by means of fluorescence Ca2+ imaging and pharmacological tools. Cells positively identified by the Co2+-labelinig assay resided primarily in the marginal zone and subplate of young fetuses and became more widely distributed throughout the cortex as the fetus matured. The majority, >80%, of these Co2+-positive cells were neurons, exhibiting immunoreactivity with the neuronal marker NeuN. The proportion of neurons that were Co2+-positive increased from ≈25% to ≈60% as the rat fetus grew into adulthood. In contrast, less than 20% of nonneuronal cells were Co2+-positive. Of the Co2+-positive neurons, 15%–31% exhibited GABA immunoreactivity and nonpyramidal-shaped cell bodies; these were presumably GABAergic neurons. Most of the remaining non-GABAergic/Co2+-positive neurons had pyramidal-shaped cell bodies and were presumably excitatory principle neurons. Around 70% of GABAergic neurons in the cortex were Co2+-positive. Furthermore, in the cortex of neonatal rats the Co2+-positive neurons were found to be more susceptible to kainate toxicity than the Co2+-negative cells. The Co2+-positive neurons in the subplate of neonatal rats were more vulnerable to kainate toxicity than their counterparts in the remaining cortical areas. Together, the widespread distribution and distinct susceptibility to excitotoxicity of CP-AMPAR-expressing neurons suggest that they play various important roles in the development and physiology of the rat cerebral cortex. J. Comp. Neurol. 518:75–91, 2010. © 2009 Wiley-Liss, Inc.