The cellular and laminar distributions of the alpha-amino-3-hydroxy-5- methyl-4-isoxazole propionate (AMPA) receptor subunits GluR1-4 have been investigated in the cerebral cortex of adult rats by in situ hybridization with 35S-labeled cRNA probes and by immunocytochemistry with subunit-specific antibodies. In sections incubated with the GluR1-4 antisense probes, specific hybridization signal was observed in many but not all cortical cells. Experiments with in situ hybridization and antibodies to glial fibrillary acidic protein (GFAP) showed that percentages of GFAP-immunoreactive cells labeled by the GluR1-4 probes were 20%, 9.4%, 8.2%, and 57.3%, respectively. A semiquantitative evaluation revealed that about 56% of cortical neurons contained the GluR1 subunit, 80% the GluR2, 63% the GluR3, and 44% the GluR4. The number of grains associated with every neuron was determined from sections exposed for 15 days, the background level was subtracted, and labeled neurons were divided into four groups: A (< or = 10 grains), B (11-20 grains), C (21-30 grains), and D (> 30 grains). The number of neurons belonging to each of these groups was then evaluated for their occurrence in each cortical layer. Immunocytochemistry with subunit-specific antibodies showed that 1) GluR1-immunoreactive neurons were mostly layers V and VI nonpyramidal neurons; 2) GluR2/3-immunoreactive neurons were more numerous in layers II-III and V-VI, and most of them were pyramidal; and 3) GluR4-positive cells were the least numerous, and they were either neurons (pyramidal and nonpyramidal) or astrocytes. These observations indicate that cortical neurons exhibit a remarkable degree of heterogeneity with regard to both the composition and the number of AMPA receptors and suggest that this diversity might be correlated with the functional attributes of neurons receiving glutamatergic afferents and with the physiological features of corticifugal neurons.