Transport systems involved in proximal tubule HCO3- reabsorption were examined in disaggregated renal cortical tubules from rabbits with metabolic alkalosis. The acid-bse disorder was induced by 1st treating the animals with furosemide, and then maintaining them on low Cl--high HCO3- diets. On this regimen, the rabbits had increases in blood pH and total CO2 values and decreases in serum K+ concentrations. Urine Cl- concentrations were < 15 mEq/l in all cases. Na+-H+ exchange was evaluated by incubating tubules in rotenone in an Na+-free medium to deplete them of Na+ and ATP. Then the tubules were resuspended in media containing 65 or 12.5 meq/l Na+ at either pH 7.1 or pH 7.6. The rise in cell pH estimated by dimethadione distribution was taken as a measure of Na+-H+ exchanger activity. At the high incubation pH, Na+-H+ exchanger activity appeared to be the same in tubules taken from alkalotic rabbits compared with those prepared from normal rabbits. At the low incubation pH, the activity of this transport system appeared to be depressed by 40%-50% in alkalosis, with kinetics that suggested a decreased Vmax for the exchanger. Na+-independent H+ transport, presumably reflecting activity of an H+-ATPase, was evaluated by preincubating tubules in a Na+-free medium in the presece of ouabain, and then sequentially exposing them to an removing them from a solution containing 20 mmol/l NH4Cl. The alkalotic tubules demonstrated rates of acid extrusion, estimated by the rate of cell pH recovery, that were .apprx. 20% greater than controls, an increase in H+ transport that is unlikely to be able to offset any significant diminution of Na+-H+ exchanger activity. The maintenance of metabolic alkalosis is not related to an enhancement in proximal tubule H+ transport mediated by the Na+-H+ exchanger.