Rats develop metabolic acidosis acutely after exercise by swimming. Renal cortical slices from exercised rats show an increase in both ammoniagenesis and gluconeogenesis from glutamine. In addition, plasma from the exercised rats also stimulates ammoniagenesis in renal cortical slices from normal rats. In exercised rats renal phosphate dependent glutaminase shows a 200% activation when the enzyme activity is measured at subsaturating concentration of glutamine (1 mM) while only an increase of 12% in V max is observed. When kidney slices from normal rats are incubated in plasma from exercised rats an activation of phosphate dependent glutaminase is obtained with 1.0 mM (100%) but not with 20 mM glutamine as substrate. This activation of phosphate dependent glutaminase at subsaturating levels of substrate may indicate a conformational change in PDG effected by a factor present in the plasma of exercised acidotic rats.