Abstract Hepatic microsomal drug metabolizing enzymes induced in rats by p, p′-DDT reach peak activity 5 days after injection, and return to control values on the 15–19th day. Reinduction with different characteristics occurred and persisted for 50 days. Massive doses are used to produce induction in experimental animals. When smaller amounts similar to those given to man therapeutically were used, it was difficult to assess if hepatic induction occurred. There was some evidence of induction after giving diphenylhydantoin, acetylsalicylic acid, and the environmental agent pyrene. Some inhibitory effects were observed after giving phenacetin and trifluoperazine. Administration of DDT and phenobarbitone per os (p.o.) increased nitroanisole demethylase activity in the small intestine. Hepatic microsomal induction produced by phenobarbitone and DDT was independent of the route of administration, and concurrent dosage did not affect the degree of induction. The effect of induction, in relation to combined exposure to pesticides, drugs and other agents, is discussed with regard to DDT storage levels in patients on anticonvulsant therapy.