We have analysed the lineage of olfactory receptor neurons using a replication-incompetent retrovirus injected beneath the olfactory epithelium of young rats. There are two major types of clusters of infected cells seen at 5--40 days after infection: (i) horizontal basal cells (HBCs); (ii) variable numbers of globose basal cells (GBCs), and immature and mature sensory neurons. Olfactory nerve lesion increased the frequency of the globose/sensory neuron clusters, as well as the number of cells/cluster, but did not change the number of HBC clusters or cells/cluster. No clusters contained sustentacular cells. These data indicate that, at least in young rats: (i) HBCs are not precursors of olfactory neurons; (ii) there is a lineage path from GBCs to mature neurons; and (iii) sustentacular cells arise from a separate lineage.