I n patients with chronic renal failure, NaHCO3 therapy may correct or prevent acidemia. It has been proposed that the NaHCO3 required will not result in clinically significant Na retention comparable to that from similar increases in NaC1 intake. In each of ten patients with chronic renal failure, creatinine clearance (Ccr) range 2.5-16.8 ml/min, on an estimated 10-meq Na and C1 diet, electrolyte excretion was compared on NaHCO3 vs NaC1 supplements of 200 meq/day. Periods of NaHCO3 and NaC1 (in alternate order for successive patients) lasted 4 days, separated by reequilibration to base-line weight. Mean +/- SEM excretion (ex) of Na, C1, and HCO3 and deltaCcr and deltaweight (day 4-1) are compared below for the 4th day of NaC1 vs. NaHCO3 intake. Mean Ccr +/-SEM on day 4 of NaC1 and NaHCO3 were 10.8 +/-1.6 and 9.0 +/-1.4 ml/min, respectively (P less than 0.02). Mean systolic blood pressure (but not diastolic) increased significantly on NaC1 (P less than 0.05). No significant blood pressure changes were seen on NaHCO3. Net positive HCO3 balance occurred on NaHCO3 as indicated above and reflected a rise in mean serum HCO3 from 19 to 30 meq/liter (day 1 vs. 4) (P less than 0.01). Mechanisms for the greater excretion of Na on NaHCO3 may relate to C1 wasting as noted above on low C1 intake and limited HCO3 reabsorptive capacity. Thus, Na excretion by day 4 was greater on NaHCO3 than on NaHCO3 did Na excretion near intake (210 meq/day).