Four children with severe cyanotic congenital heart disease and markedly reduced effective pulmonary blood flow were studied by cardiac catheterization. All children were severely hypoxic and acidotic, while the sytemic arterial pCO2 was relatively normal. These factors tended to maintain respiratory activity at a high level. However, due to the decreased pulmonary flow, respiratory compensation for the acidosis was ineffective. The chronic, uncompensated metabolic acidosis and hypoxemia suggested that (1) anaerobic tissue metabolism is significant, and (2) renal function may be compromised, in children with severe cyanotic congenital heart disease.