Cytoplasmic vesicles, resembling synaptic vesicles of efferent nerve fibers,* are a distinctive feature of sensory nerve fibers that innervate glomus cells of the carotid body (Biscoe et al, 1970; Kobayashi & Uehara, 1970; McDonald & Mitchell, 1975). Ultrastructural studies have shown that some vesicles cluster at synaptic junctions where sensory nerves are presynaptic to glomus cells (McDonald & Mitchell, 1975; Verna, 1979; Pallot, 1987). This finding has led to speculation that substances from sensory nerves influence glomus cells (McDonald & Mitchell, 1975). Indeed, the phenomenon of “efferent inhibition” (Neil & O’Regan, 1969, 1971), in which electrical stimulation of the carotid sinus nerve inhibits chemoreceptor firing, has been interpreted as an efferent action of sensory nerves on glomus cells (McDonald & Mitchell, 1981). Although there are other interpretations of the mechanism underlying efferent inhibition (Biscoe, 1971; Sampson et al, 1976; O’Regan, 1977, 1981; Acker & O’Regan, 1981), the morphological evidence that sensory nerves are presynaptic to glomus cells is compelling.