We studied 10 patients during acetate and 10 patients during bicarbonate hemodialysis to assess changes of minute ventilation; oxygen consumption (VO₂); and carbon dioxide production (VCO₂) as well as pO₂, pCO₂ and pH. We also measured the extent of pulmonic shunting by administering 100% O2. Our studies revealed that VO₂ increased significantly during acetate dialysis, while it decreased slightly during bicarbonate dialysis. Since VCO₂decreased with both baths, the respiratory exchange ratio (R) decreased during acetate dialysis but did not change during bicarbonate dialysis. By the alveolar gas equation, these changes in R could account for a difference in alveolar PO₂ and consequently arterial pO₂ The fact that pO₂ fell during bicarbonate dialysis may relate to decreased minute ventilation paralleling decreases in VCO₂. The degree of intrapulmonic shunting was not altered during dialysis with either bath. We conclude that hypoxemia during dialysis relates to decreases in minute ventilation and that a greater decrease during acetate dialysis is a consequence of enhanced VO₂ and its effect on R. Bicarbonate dialysis does not increase VO₂.