The distribution of the [3H]alpha-amino-3-hydroxy-5-methylisoxzalepropionic acid (AMPA) receptor subunit GluR4 was studied in frontal, parietal and temporal cerebral cortex, subcortical white matter and corpus callosum of neonatal, immature and mature rats. In 1- to 2-day-old rats, a few oligodendrocyte progenitors and amoeboid microglia in the supraventricular part of the corpus callosum were immunolabelled for GluR4. At 7 to 10 days, the number of amoeboid microglia and oligodendrocyte progenitors in white matter increased; many neurons in cortex, including pyramidal neurons, were also moderately labelled for GluR4. The pattern of GluR4 immunostaining in 14-day-old rats was different from that in 7- to 10-day-old rats, but similar to the adult, in that there was no immunoreactivity in microglia and oligodendrocyte progenitors in subcortical white matter. A proportion of non-pyramidal neurons in cortex were moderately labelled, while some pyramidal neurons were lightly labelled. A population of small glial cells with features of oligodendrocyte progenitors were densely labelled in cortex.