We examined the expression of basic helix-loop-helix transcription factors NSCL1 and NSCL2 in the olfactory epithelium (OE) and the vomeronasal organ (VNO) during development. As detected by in situ hybridization, at embryonic day (E) 10 NSCL1 was weakly expressed in the entire olfactory placodes. From E12 to postnatal day (P) 3, NSCL1 was expressed in olfactory receptor neurons (ORNs) and receptor neurons of the VNO. The expression pattern of NSCL2 was similar to that of NSCL1. By Northern blot analysis, strong expression of NSCL1 was detected in the OE from E12 to P7, but the expression there was low in the adult (P35). NSCL2 mRNA was detected in the E12 and P1 OE, but its level was very low in the P7 and adult OE. The spatial pattern of expression suggests that NSCL1 and NSCL2 contribute to the maturation of ORNs (VNO receptor neurons) or maintenance of their differentiated state. Moreover, the temporal pattern of expression suggests that NSCL1 and NSCL2 may function during development rather than in the adult stage.