GHANBARI–NIAKI, A., F. DÉSY AND J.-M. LAVOIE. Effects of phosphate injection on metabolic and hormonal responses to exercise in fructose-injected rats. PHYSIOL BEHAV 67(5) 747–752, 1999.—The purpose of the present study was to evaluate the effects of an intraperitoneal injection of sodium phosphate on the metabolic and hormonal responses to exercise. Fructose-injected rats were either injected with sodium phosphate (Na₂HPO₄) or NaCl, either in a fed or in a food-restricted state (24 h), and evaluated at rest or after a 30-min exercise period (26 m/min; 0% grade). Liver ATP, phosphate (Pi), and glycogen concentrations were, on the whole, significantly (p < 0.05) higher in Na₂HPO₄ than in NaCl groups. Exercise resulted in a significant (p < 0.01) decrease in liver ATP and glycogen levels in fed and food-restricted rats whether injected with NaCl or Na₂HPO₄. Exercise, after NaCl and Na₂HPO₄ injection, resulted in a significant (p < 0.01) increase in liver phosphate and Pi/ATP ratio, and in a decrease in glucose and an increase in glucagon levels in food-restricted rats only. The normal exercise-induced increase in plasma FFA, glycerol, and norepinephrine levels (p < 0.05), observed in both fed and food-restricted NaCl-injected rats, was abolished by the injection of phosphate. The data are in line with the new concept that in addition to blood glucose levels, the increase in liver Pi/ATP ratio could also contribute to the increase in glucagon response during exercise.