The proximal tubule defends the body against acid challenges by enhancing its production and secretion of ammonia. Our previous studies demonstrated an enhanced ammoniagenic response of the proximal tubule to angiotensin II added to the lumen in vitro after an in vivo acid challenge. The present study examined the effect of NH₄Cl acid-loading in vivo on renal cortical type 1 angiotensin II (AT1) receptor expression, the effect of low pH on AT1 receptor expression in a proximal tubule cells in culture and their response to angiotensin II. A short-term (18 h) NH₄Cl load resulted in increased renal cortical AT1 receptor mRNA expression and increased brush border membrane AT1 receptor protein expression levels. Changing the cell culture pH from 7.4 to 7.0 for at least 2 h increased cell surface expression of AT1 receptors and enhanced the stimulatory effect of angiotensin II on ammonia production rates. The enhanced ammoniagenic response to angiotensin II and the early enhancement of cell surface expression induced by exposure of the cultured proximal tubule cells to pH 7.0 were prevented by colchicine. These results suggest that, after acid challenges, the enhanced ammoniagenic response of the proximal tubule to angiotensin II is, in part, mediated by increased AT1 receptor cell surface expression and that the enhancement of receptor expression plays an important role in the early response of the proximal tubule to acid challenges.