The influence on renal ammoniagenesis of a high potassium diet and also of acute manipulation of ambient potassium concentration was investigated using both the isolated perfused rat kidney and incubated renal cortical tubules. Ingestion of a high potassium diet for 1 wk resulted in potassium adaptation but had no effect on ammonia production by the isolated kidney perfused with physiologic concentrations of glutamine. By contrast, perfusion with a high ambient potassium concentration (8.0-9.3 mM) significantly increased renal tissue potassium levels and concomitantly reduced the rate of ammonia formation by 30% in comparison with perfusions at a normal potassium concentration. NH3 production by tubules incubated with 1 mM glutamine was also decreased at a K+ concentration of 9.0 mM. Ammonia production was unchanged when kidneys were perfused with a potassium concentration of 2.0 mM despite a 16% decrease in renal tissue potassium levels, but ammonia production by tubules incubated in 2 mM K+ was slightly less than in control incubations at 5.0 mM. Thus, unlike earlier in vitro studies with outer medullary slices, these studies do not support the hypothesis that an adaptive change in ammoniagenesis results from a high dietary potassium intake. However, a high ambient and renal intracellular potassium concentration can depress ammonia production. Although potassium depletion causes an adaptive increase in ammonia production, a decrease in ambient potassium concentration does not increase ammoniagenesis. Accordingly, both a potassium surfeit and deficit can modify renal ammonia production, but the mechanisms involved appear to differ.