Continuous exposure of adult male Wistar albino rats to a barometric pressure of 380 mm Hg (equivalent to an altitude of 5500 m) for 28 days causes a marked increase in adrenal gland weight due to hyperplasia of both adrenal cortex and medulla. The size of the cortical and medullary components of the gland relative to each other is unaffected; in both hypoxic and control animals the cortex occupies approximately 85 per cent of the overall volume of the gland. Cortical hyperplasia is consistent with the elevated levels of ACTH which have previously been shown to occur during hypoxia. Medullary hyperplasia under such conditions presumably reflects a continuing increased requirement for catecholamines in an hypoxic environment. The morphology of these adrenal medullae bears a striking structural resemblance to some phaeochromocytomata. Adrenal medullary hyperplasia does not appear to have been described previously under such conditions and may be an important effect of prolonged severe hypoxia.