Abstract--Metabolic alkalosis is a common feature of hypokalemic hypertensive syndromes associated with angiotensin II excess. The alkalosis-generating effect of angiotensin II is usually ascribed to its stimulatory effect on aldosterone secretion, a hormone that upregulates collecting duct hydrogen ion secretion. We studied the effect of angiotensin II infusions on the expression of B1 and a4 protein, subunits of the renal H⁺-ATPase in adrenalectomized rats. Adrenalectomized rats were given either angiotensin II or vehicle for 7 days via osmotic mini-pumps. H⁺-ATPase B1 protein expression was evaluated by Western blot analysis in isolated medulla and cortex plasma membrane preparations from one kidney, whereas the contralateral kidney was used for immunostaining. By Western blotting, the relative abundance of B1 protein was 2-fold higher in renal medulla membranes from rats with intact adrenal glands (sham surgery) than from adrenalectomized rats (219±47%, n=12; P<0.05). In contrast to renal medulla, adrenalectomy did not significantly alter the relative abundance of B1 protein in renal cortex. Angiotensin II also did not significantly alter the relative levels of B1 protein in the cortex, but it increased it significantly in renal medullary membranes (231±56%, n=8; P<0.005). Moreover, enhanced H⁺-ATPase B1 subunit protein immunoreactivity was found in medullary collecting duct segments of rats infused with angiotensin II. In contrast to B1, expression of a4, another subunit of the H⁺-ATPase was not altered by adrenalectomy or angiotensin II. We conclude that adrenalectomy decreases whereas angiotensin II increases H⁺-ATPase B1 subunit expression in medullary, but not in cortical collecting ducts. By increasing the relative abundance of the B1 subunit of H⁺-ATPase in the collecting duct, angiotensin II excess may lead to increased hydrogen ion secretion and thus metabolic alkalosis--a common feature of hypertensive syndromes associated with angiotensin II overactivity.