1. Glycogen is accumulated in the liver of rats given a histidine-excess diet. 2. In order to know the regulatory point, the activities of eight hepatic key enzymes in the carbohydrate metabolism were measured after feeding either a basal or a histidine-excess diet for 0, 3 and 7 days. 3. The active glycogen synthase level was about 2.6- and 3.0-fold higher after feeding for 3 and 7 days, respectively, but the total activity of this enzyme was not affected after feeding for 3 days and was increased after feeding for 7 days. 4. The active glycogen phosphorylase, glucose 6-phosphatase and pyruvate kinase activities were significantly decreased, and fructose 1,6-bisphosphatase activity was increased on a histidine-excess diet for 3 days. 5. Thus, of these enzymatic changes, mainly the enhancement of active glycogen synthase might lead to the substantial accumulation of hepatic glycogen. 6. The in vivo incorporation of radioactive glucose into liver glycogen was stimulated in rats given a histidine-excess diet.