Administration of the synthetic glucocorticoid dexamethasone to adrenalectomized rats increased Na+/H+ exchange activity in isolated renal brush border membrane vesicles. Treatment altered the initial rate of Na+ uptake by increasing Vmax (19.90 +/- 2.17 vs. 27.32 +/- 1.50 nmol.mg protein-1.5 s-1) and not the apparent affinity KNa+ (8.33 +/- 1.11 vs. 7.94 +/- 1.60 mM). Dexamethasone treatment resulted in a proportional increase in 1 mM Na+ uptake at every intravesicular pH measured. When these data were analyzed by the Hill equation, it was found that dexamethasone treatment did not change the apparent number of H+ binding sites (1.24 vs. 1.26) or the [H+]0.5 (0.33 vs. 0.32 microM) but increased the apparent Vmax (0.98 vs. 0.55 nmol.mg protein-1.2 s-1). It was also found that dexamethasone injections of 60 micrograms/100 g body wt resulted in maximum stimulation of exchange activity and that a significant increase in amiloride-sensitive Na+ uptake was detected within 12 h after a single dose of dexamethasone.