In order to examine the effect of different dialysates on hemodialysis-induced hypoxemia, 6 stable patients with chronic pulmonary disease (CPD) were compared with 8 control subjets using, alternately, acetate and bicarbonate in the dialysate. These patients were also studied during acetate dialysis and 2 liters/min of nasally administered oxygen. In the control group the presence of hypoxemia with acetate and its absence with bicarbonate suggests that the major mechanism responsible for hypoxemia is hypoventilation. In CPD patients the higher hypoxemia with acetate which bicarbonate analysis did not prevent suggests that a superimposed mechanism may be an important factor in hypoxia aggravation. In these patients hypoxemia can be prevented easily by increasing the inspired oxygen concentration.