In recent years it has become increasingly clear that the kidney's regulation of volume homeostasis via its control of salt and water reabsorption is inextricably linked with its regulation of hydrogen ion secretion. The purн pose of this review is to outline some of the processes that control hydrogen ion secretion both in the proximal and distal nephron and to detail how these functions are related to volume regulation. This report further outн lines the difficulty sometimes encountered in assessing hydrogen ion secreн tory capacity. Such difficulties arise because salt and water reabsorption may influence parameters usually thought to reflect hydrogen ion secretory ability only. In other words, the demands of volume regulation, both on the bulk reabsorption of salt and water in the proximal tubule and on the reabsorption of salt in the ascending limb of the loop of Henle with its concomitant influence on water transport in the collecting duct, may alter tests of urinary acidification so that they seem to indicate acidification defects when none may actually exist.....