The distribution of cells expressing calcium-permeable alpha-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 Co(2+)-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 Ca(2+) imaging and pharmacological tools. Cells positively identified by the Co(2+)-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 Co(2+)-positive cells were neurons, exhibiting immunoreactivity with the neuronal marker NeuN. The proportion of neurons that were Co(2+)-positive increased from approximately 25% to approximately 60% as the rat fetus grew into adulthood. In contrast, less than 20% of nonneuronal cells were Co(2+)-positive. Of the Co(2+)-positive neurons, 15%-31% exhibited GABA immunoreactivity and nonpyramidal-shaped cell bodies; these were presumably GABAergic neurons. Most of the remaining non-GABAergic/Co(2+)-positive neurons had pyramidal-shaped cell bodies and were presumably excitatory principle neurons. Around 70% of GABAergic neurons in the cortex were Co(2+)-positive. Furthermore, in the cortex of neonatal rats the Co(2+)-positive neurons were found to be more susceptible to kainate toxicity than the Co(2+)-negative cells. The Co(2+)-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.