Induction of immediate-early gene expression, in particular c-fos, can be used to map neural activity in many brain areas, including the olfactory system. By making use of the resolution provided by cellular localization of c-fos mRNA or Fos protein, those neurons activated by a particular odor stimulus can be identified. Odor presentation to awake rats increases c-fos expression by bulb neurons located in discrete portions of the glomerular layer and in the underlying mitral and granule cell layers. The translaminar distribution of co-ordinately activated cells corresponds to the 'functional unit' predicted by the synaptic organization of the bulb, and the distribution of these units throughout the bulb as a whole differs for different odors. The bulbar pattern of activity is spatially altered by changes in odor intensity and during the course of postnatal development. These findings support the idea that distributed patterns of odor-induced neuronal activity contribute to the encoding of olfactory information. Moreover, the role of c-fos in the transcriptional regulation of other genes suggests a mechanism whereby odor experience can lead to long-term changes in the olfactory system.