Recent serial ultrathin section studies have shown that a single kind of nerve fiber innervates chief cells of the carotid body through various types of endings. These nerve endings have been described in previous papers as being vesicle-rich, mitochondria-rich or small and large calyciform; these endings occurred in en passant and bouton forms. The intracranial section experiments of the glossopharyngeal nerve seem to support the view that the nerve fiber innervating chief cells is sensory, the soma of which is located in the inferior (petrosal) ganglion, although conflicting data still exist. Two types of synapses have been found at the sites of apposition between nerve fibers and chief cells: one, named efferent or type 1 synapse, in which chief cells are postsynaptic; the other, named afferent or type 2 synapse, in which chief cells are presynaptic. The serial ultrathin section study has shown that a single nerve fiber forms both types, with the latter type predominating. The possibility is strongly suggested that most of efferent synapses on the chief cells are formed by sensory fibers which form numerous afferent synapses, but not by proper efferent fibers. The similarity between chief cells and SIF cells in the autonomic ganglia is suggested in terms of synaptic relations with neuronal elements.