(1) The size and weight of the carotid bodies have been found statistically significantly increased in 42 subjects from high altitude (14,350 ft) compared to 42 from sea level. (2) This increase is due to hyperplasia of glomic type I cells, which arrange themselves forming larger clusters and lobules at high altitudes, and in some degree to vascular engorgement. (3) Intense vacuolation of the chief cells is a histological feature of the high altitude specimens. (4) It is apparent that at high altitudes the size and weight of the carotid bodies increase with age. (5) The stimulus of chronic hypoxia is the most likely cause to explain the differences found.