The extent of mRNA editing of the kainate receptor subunit GluR5 was evaluated in tissue samples taken from the cerebral cortex, hippocampus and cerebellum of rat brain and in cortical neurons held in tissue culture, by PCR amplification of GluR5 cDNA across the edited base and restriction analysis of the amplification product with Bbv 1. Samples were taken from embryonic brains of rats at day 21 of gestation and from brains 4 days, 25 days and 3 month after birth. Cortical neurons were isolated from the tissue at day 19 of gestation and kept for 2 or 8 days in culture. The extent of editing was sharply upregulated during development in all brain structures studied. In the cortex and hippocampus the extent of editing exhibited already the adult state 4 days after birth. In the cerebellum, in contrast, the extent of editing was still 42 +/- 11.4% 25 days after birth but 82 +/- 6.2% in the adult state. In neurons held in tissue culture for up to 8 days, upregulation of editing did not take place. It is concluded that GluR5 editing is differently regulated in different brain structures and that the developmental changes observed in vivo are blocked when cells are kept in vitro.