In continuation of our pharmacological research on the in vivo induction or inhibition of the drug metabolizing enzymes system, a series of 32 derivatives of indole bearing aryl substituents in position 2 and 3 were investigated in regard to their effect on the in vivo synthesis of zoxazolamine hydroxylase in the young rat. Most fo the compounds examined proved to be inducers of this enzyme; however, as is the case with polycyclic carcinogens, the inducing activity was found to be significantly dependent on molecular structure.