The tissue localization of the G-protein y-subunit, Gy8, that is specifically expressed in the olfactory and vomeronasal neurons, was studied in rats at different ages: embryonic day 16, postnatal days 1,7, 14 and 35, and adult. G8 appears to be a specific marker of the immature olfactory and vomeronasal neurons. Its distribution differs from that of Golfα, a G-protein α-subunit which is predominantly expressed in mature olfactory neurons. G8 immunoreactivity indicates that an undifferentiated organization of the olfactory epithelium persists up to 3 weeks of age, though neonates possess a functional sense of smell. Gy8 accumulates at the highest levels in the axons of the developing olfactory neurons 2 weeks after birth (postnatal day 14). Moreover, up to postnatal day 14, Gy8–positive neurons are present in the region of the olfactory and vomeronasal epithelium, where they are not observed in later life. In the olfactory epithelium and in the bulb, Gy8 expression becomes weaker and patchy with increasing age, suggesting that the process of continuous regeneration of olfactory neurons occurs in discrete areas. G8–enhanced expression following axotomy indicates that this system is potentially active throughout life. Conversely, in the vomeronasal epithelium Gy8 expression persists virtually unmodified in the adult. This indicates that the degree of differentiation may differ between olfactory and vomeronasal neurons.