The present studies were designed to assess the handling of ammonium (NH+4) by the proximal tubule during acute metabolic acidosis (AMA). After tubule fluid collections were obtained with micropuncture techniques and in situ pH was determined near the end of the proximal tubule, 0.2 N HCl was infused intravenously at 17 microliter X min-1 X 100 g body wt-1. Thirty to sixty minutes later, samples were obtained and pH measurements were made near the previous micropuncture sites. During AMA, urine pH fell and total acid excretion doubled due to an increase in NH+4 excretion from 581 +/- 63 to 1,153 +/- 61 nmol X min-1 X g kidney wt-1 (P less than 0.001). Acid excretion did not change in time controls. Tubule fluid NH+4 rose from 2.17 +/- 0.15 to 3.45 +/- 0.24 mM during acid infusion (P less than 0.001) and its delivery to the end of the proximal tubule nearly doubled (67.8 +/- 6.3 vs. 33.9 +/- 2.9 pmol X min X g kidney wt-1 before acid infusion, P less than 0.001). This increase in delivery during AMA was due to enhanced ammonia (NH3) entry into the proximal tubule. In situ pH determined near the end of the proximal tubule averaged 6.94 +/- 0.04 before acid infusion and did not change afterwards (6.87 +/- 0.05). These data are consistent with the hypothesis that in AMA the increase in NH+4 excretion is due primarily to an increase in the cortical production of NH3.