Decreased ammonium (NH4+) excretion is associated with hyperkalemia. To determine if potassium could directly influence renal ammonia production, we investigated ammoniagenesis by rat and canine renal cortical tissues in vitro at different potassium concentrations. Renal tissue from normal and acidotic rats and normal dogs incubated in glutamine, lactate, and 7 to 10 mEq/liters of potassium or 25 mEq/liters of potassium produced significantly less ammonia than slices incubating in glutamine, lactate, and 4 to 5 mEq of potassium. Glutamate accumulation, which follows glutamine deamidation, did not decrease and even increased at 25 mEq/liters of potassium. With glutamine as the sole substrate, decreased ammoniagenesis was seen only at higher potassium concentrations (greater than 16 mEq/liters) than when lactate was also present. The depression to glutamine ammoniagenesis by high concentrations of potassium was partially obliterated in an anaerobic environment. When glutamate replaced glutamine as the precursor, renal ammonia produced by slices in 7 and 25 mEq/liters was again significantly lower than by slices incubating in 4 mEq/liters. We blocked glutamine synthesis by rat kidney slices with dl-methionine dl-sulfoximine when glutamate was the renal ammonia precursor. This essentially allows glutamate deamination to produce ammonia. Potassium depressed glutamate deamination significantly at 7 mEq/liters (decreases 13%) and at 25 mEq/liters of potassium (decreases 35%) as compared to 4 mEq/liters. The above findings are consistent with a major depressive effect of in vitro potassium on glutamate deamination in rat and canine kidneys. Other evidence, especially from rat tissue studies, suggests that potassium also may affect glutamine deamination directly. Rat kidney slices incubating in the high potassium medium of 7 mEq/liter or greater also consumed less oxygen in the presence of glutamine (P less than 0.01), oxidatively decarboxylated less glutamine (P less than 0.02) and produced less glucose from glutamine (P less than 0.01).