AbstractGlucocorticoid administration produces an increase in precursor incorporation into rat liver glutamic‐alanine transaminase in the whole animal and in liver preparations incubatedin vitro. The augmented incorporating ability was present in both the microsomal and cell sap fractions. The effect bothin vivoandin vitrowas specific, with little or no increase observed in the total soluble protein pool of the cell. From these results, together with information obtained from measurements of the biological half‐life of the enzyme, it is concluded that the increase in tissue level of the enzyme consequent to glucocorticoid administration results from an increased rate of biosynthesis. The possible role of environmental control of enzyme levels in cells through effects on degradation rates is discussed. The increased rate of precursor incorporation into liver glutamic‐alanine transaminasein vivowas aleady at a maximum value 12 hours after initiation of hormone administration, in contrast to the enzymelevelwhich had not increased by 12 hours and reached a peak only after 5 days. These findings suggest the possibility that there is a simultaneous induction by glucocorticoids of all the liver enzymes responsive to the ho