The effect of actinomycin D on the course of induction of tryptophan pyrrolase by hydrocortisone sodium succinate has been investigated in normal male Sprague-Dawley rats. In contrast to work reported in the literature, actinomycin D did not enhance tryptophan pyrrolase activity when administered during the course of hydrocortisone induction of the enzyme. It was either without effect, or was inhibitory. This finding is in agreement with current concepts on the mechanism of action of actinomycin D, and makes unnecessary the postulation of a repressor operative at the level of mRNA translation for the control of this enzyme.