A standard acid loading test was used to assess the buffering capacity in 9 hemodialysis (HD) and 25 continuous ambulatory peritoneal dialysis (CAPD) patients. There was a significant increase in H+ concentration and a decrease in plasma bicarbonate levels after the acid loading in all patients; however, PaCO2 did not change significantly. CAPD patients tolerated the acid load better, at least in the first 2 h, than HD patients. The brief duration of experiments did not allow us to observe differences, if any, in the recovery rates between HD and CAPD patients. CAPD patients tolerated the acid load equally, whether studied with or without fresh dialysate in the peritoneal cavity. Their tolerance to the acid load did not change with an increasing duration of the CAPD therapy. Baseline values and acid tolerance curves were similar in CAPD patients on regular Dianeal (lactate 35 mEq/l) and Dianeal PD2 (lactate 40 mEq/l). It is concluded that the buffering capacity is marginally higher in CAPD than in HD patients, although the baseline acid-base parameters were essentially identical.