Ultrastructure of a coronary chemoreceptor which causes a hypertensive reflex was examined in three normal dogs and three normal cats. The tissue is a periarterial glomoid body composed of highly vascular, grape-like clusters regularly located in the immediate vicinity of the left main coronary artery, from which it receives its blood supply. Its general histological organization resembles that of the carotid body. Morphometric analysis indicates that chief cells were about twice as numerous as sustentacular cells. The chief cells contain abundant osmiophilic dense granules and are surrounded by the processes of sustentacular cells. These dense granules resemble those of the carotid body and adrenal medulla, suggesting that they also may be involved in the production and storage of amines. Abundant nerve endings and formed synapses are associated with the chief cells. Numerous sympathetic and parasympathetic efferent nerve endings had characteristic presynaptic junctions to chief cells. In contrast the sensory afferents packed with mitochondria but with few synaptic vesicles had characteristic postsynaptic junctions to the chief cells. These findings suggest that the chief cells not only stores amines but also serves as a sensory or chemoreceptive site. © 1982.