Casein hydrolysate administration to fasted rats resulted in a biphasic response of glycogen synthase. Fifteen minutes after the protein meal, synthase R (active form) was increased. This was associated with a transient increase in hepatic glucose and glucose-6-phosphate (G6P) concentrations. Both glucose and G6P are known to stimulate synthase phosphatase activity, which would result in activation of synthase. Portal plasma insulin concentration was directly related to the amount of synthase R present. By 1 hour after the meal, synthase R activity was decreased compared with the control activity. Hepatic glycogen concentration was variable during the first 30 minutes after the meal, and then decreased progressively. Portal plasma glucagon concentration and phosphorylase a activity were elevated at all time points. The data suggest that the increased portal plasma glucagon concentration is the major hormonal signal for glycogen metabolism during the second hour following a pure protein meal. However, during the first 30 minutes glycogenolysis is attenuated, perhaps due to the transient increase in insulin and an increased intracellular G6P concentration.