Ultrastructure of the coronary chemoreceptor that causes a hypertensive reflex was studied immunohistochemically in human and canine hearts. Both cytologic and histologic features were similar in human beings and dogs, consisting of chief cells, sustentacular cells, Schwann cells, nerve fibers, blood vessels, and connective tissue. Three or four chief cells were typically surrounded by one Schwann cell, making a glomoid cluster about 20 microns across. Volume fractions of chief cells compared to capillaries were about 1:2 in dog and 1:4 in human chemoreceptors. Abundant osmiophilic dense granules filled the chief cells. Complex junctions between nerve fibers, chief cells, and sustentacular cells or Schwann cells exhibited a characteristic fine structure. Immunohistochemically, serotonin reactivity was observed mainly in the vicinity of junctions between nerve endings and chief cells, but some large granules in chief cells also stained positively. These new morphologic findings provide further support for the probable role of serotonin in the activation of the cardiogenic hypertensive chemoreflex in both human and canine hearts.