We previously described a rat olfactory receptor neuron (ORN) subpopulation [the 2A4(+) ORNs] that shows uniquely strong reactivity with antibodies to the 70-kD heat shock protein (HSP70) family of molecular chaperones (Carr et al.  J. Comp. Neurol. 348:150-160). The 2A4(+)ORNs are dispersed through zones II-IV of the olfactory epithelium (OE), and their axons project to only two or three glomeruli that are located consistently in each olfactory bulb (OB). To date, the 2A4(+)ORN subpopulation is the only cell population to show such distinct HSP70 immunoreactivity as well as the most discrete ORN subpopulation to be so labeled. The present report shows that 2A4(+)ORN neurons first appear between postnatal days 7 (P7) and P10. Initially, low cell numbers rise to a density of 0.1 2A4(+)ORNs/mm OE length by P14, plateau at 0.9 2A4(+)ORNs/mm by P49, then fall to adult values of 0.4 cells/mm. Autoradiographic birthdating indicates that almost all of these early appearing 2A4(+)ORNs are generated postnatally, in contrast to the prenatal generation of all ORN subpopulations characterized to date by their expression of olfactory receptor protein mRNAs. A developmentally related increase in the mean depth of 2A4(+)ORNs within the OE also occurs. In the OB, initial 2A4(+)axonal projections are to only two or three glomeruli, as in adults. Slight but significant rostral shifts in (+)glomerular location occur with development. The 2A4(+)ORN immunoreactivity was found to be due to expression of HSP70, the dominant stress-inducible member of the HSP70 family, rather than constitutively expressed HSC70. In addition, despite their presence in rat OE, no 2A4(+)ORNs were found in mice, gerbils, guinea pigs, or hamsters.