AbstractGlucokinase synthesis and degradation in rat liver have been studiedin vivoand in slices incubatedin vitro.Glucokinase induction in fasted rats can be obtained by administration of either glucose or insulin, although the amount of enzyme that accumulates in the latter case is small. There is an apparent lag of about 2 hours from administration of external inducer to detectable increase of glucokinase in liver. The results of delayed administration of actinomycin and puromycin indicate that formation of messenger RNA starts within 1 hour, and that completion of active enzyme rapidly follows polypeptide synthesis. An incubation system for liver slices has been developed with which glucokinase can be studiedin situfor several hours. Glucose, but not insulin, stabilizes the enzyme. Net synthesis of glucokinase has been obtained in slices of liver taken from the animal shortly after the apparent induction lag. Messenger RNA for glucokinase seems to be fairly stable; its half‐life appears to be greater than 8 hours. Actinomycin has a paradoxical effect on the disappearance of glucokinase by fasting; during the first day it prevents the normal decrease and even increases glucokinase in liver. A model of regulation of the level of glucokinase in liver is proposed in which insulin induces enzyme synthesis, high glucose concentration favors accumulation by slowing down its degradation, and glucagon prevents enzyme accumulation at a still undefined level. The rapid decrease of liver hexokinase activity in actinomycin‐treated animals is also repor