Ultrastructural examinations of the CB showed that NOS-related NADPH-d activity is present in the chemoreceptor cells (Fig. 1A). The NADPH-d reaction product appeared as small, intense, dark particles scattered in the cytoplasm of the cells. The particles were distributed most densely or tended to aggregate in the perinuclear region, the endoplasmic reticulum channels, and in between the dense-core vesicles but not in the vesicles themselves. These particles were distinguishable from ribosomes from which they were larger and more electron dense. The particles were absent in the control specimens in which B-NADPH was omitted (Fig. 1B). The presence of NOS, which synthesizes a short-lived gaseous NO messenger, may have significant implications for the CB function. NO could not only affect the autonomic and sensory nerve endings but also be a direct participant of the chemoreceptor cell signaling process. The exact role of the nitergic pathway in the CB is unclear at present.