1. 1. Starvation and the administration of glucagon exert several similar effects on the levels of liver tryptophan pyrrolase and tyrosine transaminase. The most striking one of these is the fifteenfold increase in the level of tyrosine transaminase by the synergistic action of glucagon and hydrocortisone. 2. 2. Irradiation, unlike actinomycin, inhibits the cofactor-type inductions of tryptophan pyrrolase and tyrosine transaminase and does not influence their hormone-type induction. 3. 3. An operational definition of enzyme induction is discussed. The known facts about the mechanisms of induction in animals are contrasted with the ambiguous words commonly used to describe them.