Abstract Both acute respiratory acidosis and acute metabolic acidosis stimulate NH 3 production by the isolated perfused rat kidney. This stimulatory effect is abolished if the urine is drained back into the recirculating perfusate rather than collected. To determine whether the urinary inhibitor is a cyclooxygenase product, studies were carried out using prostaglandin synthetase inhibitors. Kidneys perfused with 0.5 mmol/L glutamine and urine reinfusion were subjected to acute respiratory acidosis (30% CO 2, pH 6.8). With either indomethacin (20 μmol/L) or meclofenamate (20 μmol/L) in the perfusate, NH 3 production increased significantly in response to acute respiratory acidosis despite urine reinfusion. The increment in NH 3 production was comparable to that in studies with urine collection, indicating that a cyclooxygenase product can account completely for the urinary inhibitor. To further characterize the urinary prostaglandin inhibitor, studies were performed with both the isolated perfused kidney and renal cortical tubules. Prostaglandin E 2 (PGE 2) did not exhibit an inhibitory effect on NH 3 production with either experimental model. Prostaglandin F 2α at low doses inhibited NH 3 production in response to acute acidosis by the isolated kidney, but an effect was not apparent with higher concentrations. PGF 2α inhibited the stimulatory effect of a low pH (7.1) on NH 3 production by isolated tubules, and had no effect on ammoniagenesis at pH 7.4. Thus a prostaglandin, which is not PGE 2 and may be PGF 2α, appears to be the previously unidentified urinary inhibitor of the ammoniagenic response to acute acidosis found with the isolated perfused kidney.