Earlier experiments in vivo with adrenalectomized rats showed that for the induction of tryptophan oxygenase and tyrosine aminotransferase small doses of cortisone acetate and the corresponding substrate are necessary. We performed the similar experiments in vitro with liver slices from adrenalectomized rats which were pretreated with cortisone acetate in vivo. Studies with alpha-amanitine and cordycepin--given in vivo--showed, that the synthesis of m -RNA for tryptophan oxygenase after application of 1 mg/kg cortisone acetate lasted for less than 2 h. The results obtained in vitro were very similar to those reported earlier. Maximum activity is obtained in the presence of the substrate L-tryptophan. When adding other amino acids instead of the substrate no effect can be observed. The same is true also for actinomycin D. In the presence of puromycin and cycloheximide the enzyme activity decreases.