The release of intracellular Ca, which is involved in many neuronal functions, is regulated by the second messenger inositol 1,4,5-trisphosphate (InsP3) interacting with specific receptor. The distribution of the mRNA coding for the recently cloned InsP3 receptor was studied in the developing rat brain using oligonucleotides derived from the rat cDNA sequence and in situ hybridization. The localizations of the mRNA in the postnatal brain were exactly superimposable to that previously reported in the adult [Mailleux et al., Neuroscience, 49 (1992)577-590]. Higher mRNA levels were consistently found in the adult neurons over their postnatal counterpart. Hybridization signal was first visible in the cerebellar Purkinje cells which express dramatically higher mRNA levels of the receptor than any other neurons in the brain. In conclusion, the levels of InsP3 receptor mRNA per neuron increased with synaptogenesis. This finding suggests the occurrence during this critical developmental period of a more complex regulation of Ca fluxes, perhaps requiring higher intraneuronal levels of InsP3 receptor.