THE distribution and morphology of nitric oxide-synthesizing elements in the human olfactory bulb were studied using NADPH-diaphorase histochemistry and nitric oxide synthase immunohistochemistry. NADPH-diaphorase was detected in all olfactory fibers and groups of superficial short-axon cells, deep short-axon cells, stellate cells and abundant centrifugal fibers. Similar cell types were nitric oxide synthase immunoreactive but olfactory fibers were immunonegative. The distribution patterns of nitric oxide-synthesizing elements showed significant differences from what has been reported in the olfactory bulb of macrosmatic mammals including rodents and insectivores. These differences are likely to correlate with interspecies differences in the processing of olfactory information.