Induction of cytosolic aspartate aminotransferase (cAspAT) was observed in rat liver on administration of a high-protein diet, or glucagon and during fasting. The enzyme activity in the liver of rats given 80% protein diet or glucagon injection during starvation increased to 2- to 2.4-fold that in the liver of rats maintained on 20% protein diet, with about 2-fold increases in the levels of hybridizable cAspAT mRNA, measured by blot analysis using the cloned rat cAspAT cDNA as a probe. No increase in the enzyme was detected in kidney, heart, brain, or skeletal muscle. The activity of mitochondrial aspartate aminotransferase (mAspAT) did not increase. Induction of cAspAT was observed when glucose metabolism tended toward gluconeogenesis. The physiological function of the induction of cAspAT is considered to be to increase the supply of oxaloacetate as a substrate for cytosolic phosphoenolpyruvate carboxykinase (PEPCK) [EC 4.1.1.32] for gluconeogenesis.