Recent progress in the studies of the olfactory system, especially in the molecular biological studies, makes it one of the useful sensory model systems for understanding neural mechanisms for the information processing. In the olfactory bulb, the primary center of the olfactory system, glomeruli are regarded as important functional units in the transmission of odorant signals and in processing the olfactory information, but have been believed to be composed by only a small number of neuronal types and thus to be simple in their neuronal and synaptic organization. However, accumulating morphological data reveal that each type of neurons might further consist of several different subpopulations, indicating that the organization of glomeruli might not be so simple as it was believed. Here we describe an aspect of the structural organization of glomeruli, focusing on the heterogeneities of periglomerular neurons in mammalian main olfactory bulb.