We have recently isolated and identified a brain specific Na(+)- dependent inorganic phosphate (Pi) cotransporter cDNA (rBNPI) from a rat brain cDNA library (Ni, et al., 1994). We now report the regional and developmental expression, as well as the cellular localization, of rBNPI mRNA in the rat brain. In situ hybridization histochemistry reveals that rBNPI mRNA is selectively expressed in neuron-enriched regions of the adult rat brain, such as the cerebral cortex, hippocampus, and cerebellum. Cellular localization of rBNPI transcripts reveals expression in both pyramidal and granule neurons in these regions. By contrast, little to no hybridization signal was observed in white matter-enriched areas such as the corpus callosum. The expression of rBNPI mRNA was determined during pre- and postnatal development of the rat CNS. From embryonic day 17 to early postnatal day 10 (PND 10), there is a rather widespread but diffuse pattern of rBNPI expression in brain. During late postnatal development, however, the expression of rBNPI mRNA becomes confined to discrete populations of neurons in the cerebral cortex, hippocampus, and cerebellum. Thus, rBNPI expression is developmentally regulated and abundant levels of mRNA are found in rather discrete populations of neurons in the adult rat brain. The latter suggests that rBNPI may serve to selectively regulate intracellular Pi transport in certain neurons for either metabolic and (or) signaling events.