Acidosis and angiotensin II (ANG II) stimulate ammonia production and transport by the proximal tubule. We examined the effect of short-term (18 h) in vivo acid loading with NH(4)Cl on ammonia production and secretion rates by mouse S2 proximal tubule segments microperfused in vitro with or without ANG II in the luminal microperfusion solution. S2 tubules from NH(4)Cl-treated mice displayed higher rates of luminal ammonia secretion compared with those from control mice. The adaptive increase in ammonia secretion in NH(4)Cl-treated mice was eliminated when losartan was coadministered in vivo with NH(4)Cl. Ammonia secretion rates from both NH(4)Cl-treated and control mice were largely inhibited by amiloride. Addition of ANG II to the microperfusion solution enhanced ammonia secretion and production rates to a greater extent in tubules from NH(4)Cl-treated mice compared with those from controls, and the stimulatory effects of ANG II were blocked by losartan. These results demonstrate that a short-term acid challenge induces an adaptive increase in ammonia secretion by the proximal tubule and suggest that ANG II plays an important role in the adaptive enhancement of ammonia secretion that is observed with short-term acid challenges.