Free-flow micropuncture studies were carried out in rats to study the relationship between ammonium and potassium transport along superficial nephrons. Proximal and distal tubular fluid was analyzed for [3H]inulin, potassium, and ammonium. Ammonium was measured by a titrimetric formaldehyde method using a glass electrode to measure pH and an antimony electrode to deliver OH ions. In addition to experiments carried out under control conditions, the rats were also acutely potassium loaded, chronically potassium depleted, or given glutamine intravenously to stimulate ammonium excretion. Ammonium production along the proximal tubule was sharply enhanced in potassium depletion and, most likely, also following glutamine infusion, but it remained unchanged during hyperkalemia. Significant amounts of ammonium were lost along the loop of Henle in all experimental conditions. During stimulation of ammonium excretion by either potassium depletion or glutamine infusion the urine became alkaline. No loss or gain of ammonium beyond the late distal tubule was observed in any of the experimental conditions. With respect to potassium transport the glutamine-induced rise in urinary ammonium excretion was associated with stimulation of potassium reabsorption beyond the late distal tubule.