The pattern of respiratory compensation in chronic uremic acidosis has been delineated on the basis of an analysis of 145 determinations of the arterial acid base status of 19 patients suffering from terminal renal failure and treated by chronic hemodialysis. In the bicarbonate range studied (11–21 mEq/l) it was shown that pCO2 was linearly related to plasma bicarbonate concentration (pCO2 = 1.214 HCO-3 + 10.87), whereas blood hydrogen ion concentration rises in a curvilinear fashion with decreasing plasma bicarbonate concentration. Significance bands enabling the prediction of the physiological ventilatory response to chronic metabolic acidosis were calculated from these data. An analysis of the individual ventilatory response to metabolic acidosis was made in 10 patients. No difference was found between the slopes of the pCO2 response lines whereas the elevations of the lines were shown to vary from individual to individual. This suggests that every subject has his own threshold of sensitivity to metabolic acidosis. Between two dialyses, plasma bicarbonate concentration fell linearly as a function of time, at a rate of 1.9 mEq/l per day. The effect of dialysis on the ventilatory response to acidosis was evaluated on the basis of an analysis of 96 determinations of the arterial acid base status performed immediately after dialysis. It was shown that a 12 h dialysis did not modify the linearity nor the slope of the pCO2 bicarbonate relationship but decreased its elevation (pC02= 1.203 HCO-3+ 7.67 ± 2.4