In situ hybridization approaches have been employed to explore the onset and time course of odorant receptor expression during the prenatal development of rats. The critical phase for the maturation of the olfactory system from embryonic day (E) 12 to E18 was analysed. The onset of expression of four receptor genes (OR5, OR14, OR37 and OR124) was found between day E12 and E14. In the early phase of development (E14) a distinct receptor subtype was expressed only in a few hundred neurons; the number increased about two- to three-fold within a 2 day interval. From the very beginning, spatial segregation of receptor subtypes in distinct expression zones was observed.