Urinary citrate is an important endogenous inhibitor of calcium nephrolithiasis. Systemic acidosis increases renal citrate reabsorption (decreases urinary excretion) and hence is associated with nephrolithiasis; systemic alkali administration increases citrate excretion. We studied the mechanism by which acidosis and alkalosis alter citrate reabsorption in the proximal convoluted tubule, the predominant nephron segment reabsorbing citrate. Tubules were perfused in vitro and citrate reabsorption was measured by use of luminal [14C]citrate. Changes in solution pH were accomplished by changes in bicarbonate concentration with constant PCO2. Decreasing peritubular pH acutely from 7.4 to 7.2 caused an increase in citrate reabsorption. However, the change seen with an acid peritubular pH was abolished by additional buffering of the luminal solution. Increasing peritubular pH from 7.4 to 7.6 resulted in a fall in citrate reabsorption that was not significantly greater than a time-dependent fall in citrate reabsorption in this preparation. The effect of luminal pH on proximal citrate reabsorption was also examined directly. Decreasing perfusate (luminal) pH from 7.4 to 7.2 with constant peritubular pH increased citrate reabsorption. Increasing perfusate pH to 7.6 decreased citrate reabsorption insignificantly (0.1 less than P less than 0.2). These data suggest that luminal pH in the proximal tubule is an important determinant of alterations in citrate reabsorption with acid-base disorders. The effect of luminal pH on citrate reabsorption is probably due to a change in concentration of the transported ionic species, citrate2-.