These studies explore the distribution of putative neuroactive peptides in the human olfactory bulb. Localization of synaptophysin-, serotonin-, cholecystokinin-, substance P-, and somatostatin-like staining was examined by immunocytochemical protocols. The results provide new insights into the composition and laminar segregation of subpopulations of neurons and neuronal processes in the human olfactory bulb. The prominent synaptophysin-like immunoreactivity observed in the glomeruli of the human olfactory bulb is consistent with the notion that the density of synapses, and hence the density of synaptic vesicles, is highest in the glomeruli. Serotonin-like immunoreactivity suggested a variable innervation of glomeruli ranging from a dense tangled ball of fibers within the glomerulus to a sparse innervation by a single immunoreactive fiber. There was no evidence of serotonin-like immunoreactive cell bodies in either the olfactory bulb proper, anterior olfactory nucleus, or proximal regions of the lateral olfactory tract. Cholecystokinin-like immunoreactivity was limited to fibers found largely in the juxtaglomerular region of the glomerular layer. In the deeper layers of the olfactory bulb, cholecystokinin-like immunoreactive fibers did not show any of branching or arborization that was evident in the juxtaglomerular region. Substance P-like immunoreactivity was seen in varicose fibers distributed in all of the human olfactory bulb laminae. In addition, stained multipolar neurons were found in the area of the anterior olfactory nucleus. Somatostatin-like immunoreactivity was similar to that of substance P in that a plexus of stained fibers was found in all laminae of the olfactory bulb. Also, somatostatin-like immunoreactive cell bodies were found in the area of the anterior olfactory nucleus. However, as compared to substance P, somatostatin had a less dense plexus of immunoreactive fibers in the olfactory bulb. These results increase our understanding of the fundamental organization of the human olfactory system. The current data, coupled with prior studies, provide a foundation from which to study the cellular pathology of diseases with known olfactory system sequelae such as Alzheimer's, Parkinson's, and schizophrenia.