The role of renal glutamine synthesis for the rapid decrease in renal ammoniagenesis occurring early in the recovery phase (24 h) of metabolic acidosis was studied in rats. L-Methionine-DL-sulfoximine (MSO), an irreversible inhibitor of glutamine synthetase, depressed the renal enzyme activity by 50% but did not impair the recovery from acidosis. Since extrarenal glutamine synthesis was decreased by this manoeuvre with lowering of blood glutamine, an intravenous load of L-glutamine sufficient to elevate blood concentration to 1 mM was superimposed on the MSO treatment. The glutamine load did not increase the ammoniuria. Infusion of glutamine alone to rats recovering from metabolic acidosis for 12-24 h did not change their ammoniuria. In contrast, glutamine administration together with HCl produced a marked ammoniuric response in rats recovering from acidosis. Conversely, the administration of bicarbonate to chronically acidotic rats acutely depressed renal ammonia production. It is concluded that glutamine synthetase activity is probably not required for recovery from metabolic acidosis, and that the post-acidosis alkaline rebound occurring in the rat may play a direct role in suppressing the ammoniagenic pathway either by drastic reduction in mitochondrial permeability for glutamine or acute inhibition of intramitochondrial deamidation of this amino acid.