Disruption of the mouse gene encoding the transcription factor MASH1 leads to loss of certain classes of neurons, including receptor neurons of the olfactory epithelium (OE). Here we investigate the nature of the cell type expressing MASH1 in mouse OE by manipulating olfactory receptor neuron (ORN) neurogenesis in vitro and in vivo to alter the dynamics of neuronal production. The results indicate that MASH1 is expressed in cells of the ORN lineage, but not in ORNs themselves nor in their immediate precursors. Data on how changes in the numbers and proliferative states of MASH+ cells correlate with induced changes in overall neurogenesis strongly suggest that MASH1-expressing cells give rise to the immediate precursors of ORNs, but are not the self-renewing stem cells of the OE. The results imply that multiple progenitor stages are employed in generating ORNs and suggest that the action of MASH1 occurs predominantly at an intermediate stage.