17 subjects were carefully selected among 150 patients with COLD: all patients with one or more conditions or diseases that could alter the physiological response to chronic hypercapnia were eliminated. During a period of 13 to 28 days, the acid-base response to slow carbon dioxide changes was studied in the selected patients. Total body buffering capacity was expressed as dH+a/dPaCO2 and showed to be an inverse function of chronic PaCO2 (defined as the value of PaCO2 in clinical and acid-base steady status for each patient), according to the previous suggestions of Weiss and Dulfano and to the report of Ingram and co-workers during acute carbon dioxide changes in the same patients