A number of recent studies have suggested that the medullary and papillary collecting ducts possess a high capacity for bicarbonate reabsorption. Because 3H binding studies have suggested the presence of mineralocorticoid receptors along the papillary collecting duct and because clearance studies have suggested that aldosterone affects urinary acidification, we designed a series of clearance and micropuncture studies to examine the role of the papillary collecting duct in final urinary acidification. The studies were conducted in intact and adrenalectomized rats receiving either replacement glucocorticoids alone or glucocorticoids with acute infusion of physiological doses of aldosterone. The results show that under normal acid-base status all rats had a pH profile along the papillary collecting duct with more terminal portions being more acidic than the proximal portions. This decreasing pH profile was diminished in acidotic adrenalectomized rats but returned to normal with acute administration of aldosterone. In all cases the luminal potential difference was slightly positive and did not change with adrenalectomy or administration of aldosterone. These results demonstrate that the papillary collecting duct participates in the generation of maximal urinary pH gradient and that this process is partly influenced by aldosterone.