Sixteen adult male squirrel monkeys (Saimiri sciureus) were randomly divided into three treatment groups and one control group. Each treatment group received 10 mg/kg oral doses of diphenylhydantoin and/or chloroquine. Following sacrifice, in vitro assays for activity of liver microsomal mixed‐function oxidases were run. The assays confirmed diphenylhydantoin as a potent inducer of mixed‐function oxidases. Chloroquine administration had little affect on the enzymes assayed and did not inhibit the diphenylhydantoin induction.