The histopathology of hyperplasia of the carotid bodies was studied in 6 cases of hypoxaemia and right ventricular hypertrophy secondary to pan-acinar emphysema, and in five cases of systemic hypertension with left ventricular hypertrophy. The features of the hyperplasia were the same in the two groups. There was proliferation of sustentacular (type II) cells and compression of central cores of chief (type I) cells. It is speculated that the hyperplasia of sustentacular cells is associated in some way with the prevention of retention of sodium ions and water which characterises hypoxic cor pulmonale in "blue bloaters", systemic hypertension, and ascent to high altitude with the complications of acute mountain sickness, and pulmonary and cerebral oedema.