The distribution of vasoactive intestinal peptide (VIP)- and neuropeptide Y (NPY)-like immunoreactivities in the Sprague-Dawley rat main olfactory bulb was analyzed using the peroxidase-antiperoxidase light microscopic immunocytochemical technique. VIP-like immunoreactivity was most prominently localized within a large number of intermediate-sized neurons whose perikarya and extensively branched varicose processes remained confined to the external plexiform layer (EPL). A few small short-axon type neurons in the mitral cell layer and granule cell layer (GRL) and even fewer large neurons in the glomerular layer (GL)/EPL border region contained immunoreactivity for VIP as well. Neuropeptide Y-like immunoreactivity (NPY-I) was principally localized within sparsely distributed large multipolar neurons of the deep GRL and within axons distributed with diminishing density from deep to superficial GRL. In addition, dense NPY-I was localized within very few large superficial short-axon type neurons of the GL/EPL border region. The restricted laminar and cellular distribution of NPY-I and VIP-I suggests that both peptides may act to modulate granule cell activity, and therefore, indirectly, olfactory bulb output.