The purpose of the experiment was to compare the effects of dilantin, chloroquine and phenylbutazone on the storage of p,p'-DDT in the rat. Eleven groups of six animals were administered drugs in their feed for ten weeks. At the time of sacrifice, liver microsomal enzymes, including te O-demethylation of p-nitroanisole, oxidative detoxificatiion of EPN, azo-reductase, expoxidase, glucuronal transferase and cytochrome P-450, were determined. The fat of all animals was examined for p,p'-DDT and p,p'-DDE in the fat of the animals; however, the degree of reduction was not identical for the three drugs, and the reduction due to combination treatments was not that which would have been predicted had the treatment effects been additive. The three drugs exhibited different inductive effects on the several hepatic microsomal enzymes measured.