Serum concentrations of p,p'DDT and its metabolite p,p'DDE were estimated by electron-capture gas chromatography in a group of mentally retarded institutionalized patients receiving antiseizure and tranquilizing chemotherapy. Control patients not taking drugs tended to resemble the general population in terms of organochlorine residues, but drug takers showed greatly lowered levels of both p,p'DDE and p,p'DDT. The combination of phenobarbital and diphenylhydantoin seemed to be the most effective in depressing serum pesticides and remained so even in patients taking other drugs concurrently. P,P'DDE was found at detectable levels in all patients sampled, but the frequency of occurrence of p,p'DDT was considerably decreased among the drug takers. This was particularly true of patients receiving both phenobarbital and diphenylhydantoin concurrently. The possibility that the catabolism of p,p'DDT is induced preferentially to that of p,p'DDE by the phenobarbital-diphenylhydantoin combination is discussed.