Proliferation, differentiation and death of olfactory neurons occur continually, even in adult animals. New data suggest that growth factors regulate the rate of cell proliferation. Early growth of olfactory axons in embryonic development is accompanied by the migration of epithelial cells from the olfactory placode toward the presumptive olfactory bulb. Maturation and ciliogenesis at the dendritic end of the cell is apparently dependent on a signal(s) from the bulb. The total life span of the neuron depends on maintenance of contact with the bulb. Olfactory life span is normally variable but is curtailed substantially in the absence of the bulb.