Rats prefed a 90% glucose diet for 4 days were fed ad libitum a 90% casein diet for a period of 24 hr. The transitory elevation in pyruvate kinase activity in rats fed the 90% casein diet was not inhibited by 30 mg of 8-azaguanine administered in 2 equal doses 12 hr apart. In rats prefed for 5 days a 90% casein diet, ad libitum feeding of the 90% glucose diet increased pyruvate kinase activity and in this case the increase in activity was inhibited by 8-azaguanine. The results confirm earlier data and are consistent with the hypothesis that in rats prefed the high-carbohydrate, protein-free diet the synthesis of pyruvate kinase is limited at the level of translation, whereas in rats prefed the high-protein, carbohydrate-free diet the synthesis of the enzymes is limited at the level of transcription.