The anterior olfactory nucleus (AON) occupies a crucial position within the olfactory circuit, as it is able to influence function in nearly every major synaptic processing stage of both the ipsilateral and the contralateral pathways. Nevertheless, very little is known about the region's internal organization and circuitry. The present study provides basic quantitative and qualitative data on the morphology of several cell types within the two major regions of the AON, pars externa and pars principalis. In pars externa two types of cells are analyzed, the “classical” cell (type I), containing only apically directed dendrites with large spines, and a previously unreported cell with basilar dendrites and complex, spiny apical processes (type II). In pars principalis the characteristic pyramidal cell is described both on the basis of the depth of the cell bodies in the cell layer comprising the structure and on the basis of their radial location. Several other nonpyramidal neurons are also described. The findings provide useful basic information necessary for understanding and modeling the circuitry of the AON. J. Comp. Neurol. 518:1603–1616, 2010. © 2009 Wiley-Liss, Inc.