In a limited number of subjects having hypoxemia related to congenital malformations of the heart, no evidence was obtained of a correlation between the degree of hypoxemia or the extent of the polycythemia and the systemic blood flow. The circulatory response to exercise was characterized by a greater increase in the arteriovenous oxygen difference than in the systemic blood flow. A syncopal reaction in one patient was preceded by a greatly increased systemic blood flow and a small arteriovenous oxygen difference.