The tryptophan-induced elevation of rat liver tryptophan pyrrolase activity in vivo is associated with two consecutive processes: (a) saturation of the endogenous apotryptophan pyrrolase with its heme cofactor and (b) an increase in the amount of the protein moiety of the enzyme system which can be determined by immunological titration. Process (b) is inhibited by the administration of puromycin (an inhibitor of protein synthesis) but not by that of actinomycin (an inhibitor of RNA synthesis). The induction of tryptophan pyrrolase by cortisone involves process (b alone and is inhibited by puromycin as well as by actinomycin. These agents also inhibit the cortisone-induced elevation of the level of liver tyrosine-α-ketoglutarate transaminase. The results suggest a basic difference between the mechanism of substrate- and hormone-induced stimulation of enzyme synthesis. The level of activity of rat liver threonine dehydrase is decreased by a vitamin B6 deficient diet. The degree of saturation of this enzyme with pyridoxal phosphate and the activity measured in the presence of excess cofactor is not altered by the administration of threonine in vivo. The early postnatal accumulation of tryptophan pyrrolase, unlike that of tyrosine-α-ketoglutarate transaminase, is not inhibited by adrenalectomy or actinomycin. Observations on the ratio of holo- to apotryptophan pyrrolase during development also suggests a common mechanism underlying the developmental and substrate-induced elevation of this enzyme. The present findings exemplify two ways in which factors regulating enzyme synthesis in adult or developing tissues may operate. They may influence the number of potential synthetic sites (RNA template units) or the speed of functioning of a constant number of these sites.