1. Young rats were kept in a hypoxic chamber for 2–11 weeks and compared with littermate control animals.
2. The carotid bodies of the hypoxic rats enlarged, resembling those of men and animals living at high altitude.
3. Permanent blunting of the ventilatory response to hypoxia did not occur. Immediately on removal from the chamber, the rats, lightly anaesthetized, showed a smaller increase in ventilation during hypoxia than did control animals but this difference disappeared after 3 days' recovery in normoxia.