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The embryonic and postnatal localizations of oxytocin receptor mRNA in the developing rat brain were studied by in situ hybridization histochemistry. The hybridization signal was first detected at embryonic-day 13 in the primordium of the dorsal motor nucleus of vagus. Other positive regions progressively appeared after this time. The developmental profile of oxytocin receptor gene expression could be classified into two types; transient expression and constant abundant expression. The caudate putamen, cingulate cortex, the anterior thalamic nuclei, and the ventral tegmental area belonged to the first type. In these regions, oxytocin receptor mRNA was expressed intensely only during the early postnatal period. The regions such as the anterior olfactory nucleus, tenia tecta, some amygdaloid nuclei, piriform cortex, the ventromedial hypothalamic nucleus, subiculum, the prepositus hypoglossal nucleus and the dorsal motor nucleus of vagus showed constant expression of oxytocin receptor mRNA at high levels throughout development and in the adult. These findings concurred well with those of the ontogenic studies using receptor binding autoradiography with a ligand specific to oxytocin. Thus, the transient expression of oxytocin receptor during development was regulated at the transcriptional level in several brain regions, and oxytocin may play a role in brain development as well as in neural transmission in the mature brain.