The effect of the tricarboxylic acid (TCA) cycle precursor, pyruvate, on glutamine metabolism by isolated renal cortical mitochondria was assessed by quantitating its key nitrogen and carbon metabolites. When mitochondria from normal rats were incubated at pH 7.4, pyruvate (2 mM) inhibited ammonia production by almost completely erradicating glutamate deamination and by diminishing glutamine deamidation but to a lesser extent. Alpha KG, citrate, and malate accumulation in the incubation medium were increased dramatically reflecting the increased flux of pyruvate through the TCA cycle; the intramitochondrial concentrations of both Alpha KG and glutamate were increased. Thus, pyruvate primarily inhibits flux through glutamate dehydrogenase as a result either of an increase in Alpha KG concentration and/or a decrease in the redox (NAD/NADH) potential secondary to enhanced flux through the TCA cycle. Glutamine deamidation is secondarily inhibited, presumably due to the increased intramitochondrial concentration of glutamate. Citrate (2 mM) produced changes comparable to those observed with pyruvate. Mitochondria from normal rats incubated at pH 7.0 as well as mitochondria from rats with chronic metabolic acidosis responded to pyruvate in a fashion qualitatively similar to normal mitochondria incubated at pH 7.4. Glutamate deamination was inhibited significantly, but a high rate persisted with chronic acidosis despite the presence of pyruvate. Nevertheless, when glutamine metabolism was contrasted with normal mitochondria incubated at pH 7.4, the response to in vitro incubation in an acid pH as well as to chronic metabolic acidosis was similar quantitatively regardless of whether glutamine alone or in combination with pyruvate was present in the incubation medium.