It was previously shown that glucagon and epinephrine have additive effects on both gluconeogenic and glycogenolytic flux. However, the changes in gluconeogenic substrates may have been limiting and thus may have prevented a synergistic effect on gluconeogenesis and a reciprocal inhibitory effect on glycogenolysis. Thus the aim of the present study was to determine if glucagon has a greater gluconeogenic and a smaller glycogenolytic effect in the presence of both epinephrine and clamped gluconeogenic precursors. Two groups (Epi and G + Epi + P) of 18-h-fasted conscious dogs were studied. In Epi, epinephrine was increased, and in G + Epi + P, glucagon and epinephrine were increased. Gluconeogenic precursors (lactate and alanine) were infused in G + Epi + P to match the rise that occurred in Epi. Insulin and glucose levels were also controlled and were similar in the two groups. Epinephrine and precursor administration increased glucagon's effect on gluconeogenesis (4.5-fold; P < 0.05) and decreased glucagon's effect on glycogenolysis (85%; P = 0.08). Thus, in the presence of both hormones, and when the gluconeogenic precursor supply is maintained, gluconeogenic flux is potentiated and glycogenolytic flux is inhibited.