The area occupied by parenchymal cells, in sections comprising the entire half of the surface of the carotid body, is significantly greater in people born and living at 14,350 feet than in those at sea level. The enlargement and increase in weight of the carotid bodies observed at high altitudes can thus be attributed to hyperplasia of parenchymal tissue. The proliferated cells have the morphology of type I chief cells and display marked vacuolation and depletion of yellow-green, naturally fluorescing, bioamine containing granules. Although unimportant variations in size and weight in relation to age occurred at sea level, it was found that the magnitude of the carotid body enlargement increased with age at high altitudes. The augmented carotid body size and weight in relation to age at high altitudes are associated with progressive chemoreceptor insensitivity. The physiologic and pathologic significance of these findings is discussed.