The ultrastructure of the carotid body in chronically hypoxic rabbits.

1. The ultrastructure of the carotid body in the rabbit has been examined by electron microscopy.2. A comparison was made between the ultrastructure of the carotid bodies in sea level rabbits, in rabbits which had been exposed to hypoxia equivalent to an altitude of 6000 m for 7 days and in rabbits which always had lived at an altitude of 4000-4300 m.3. We could not detect any difference in the ultrastructure between the two groups of hypoxic rabbits.4. When the hypoxic rabbits were compared with sea level rabbits there was a marked increase in the number of dense cored vesicles and mitochondria in the type I cells in the hypoxic rabbits. The Golgi region also appeared to be enlarged in the type I cells in the hypoxic rabbits.5. The finding suggests that in the rabbit the production of amines, probably dopamine, within the type I cells is increased during prolonged hypoxia which might explain the lowered ventilatory response to hypoxia observed in human high altitude residents.6. If the carotid bodies are organs of internal secretion the finding is compatible with an increased production of a hormone produced within the type I cells.