1. The ability of the cestode Moniezia expansa and the nematode Ascaris lumbricoides var suum to metabolize foreign compounds has been assessed. 2. Both species were unable to oxidase aldrin, aniline, biphenyl, butylbenzene and nitrobenzene or to demethylate aminopyrine, and 4-nitroanisole. 3. M. expansa and A. lumbricoides var suum readily induced 4-nitroanisole, nitrobenzene, 4-nitrobenzoic acid and 4-nitrophenol to the corresponding amines. Azobenzene, dimethylaminoazobenzene, and 1,2-dimethyl-4-(4-carboxyphenylazo)-5-hydroxybenzene were also reduced. 4. Hydrolysis of esters, acetanilide, acetylsalicylic acid, aryl sulphates and aryl phosphates took place readily. However, beta-glucuronides were not hydrolysed. 5. The following reactions were not detected in either species: phosphate, sulphate, beta-glucuronide or beta-glycoside conjugation of phenolic compounds; acetylation of amino compounds, or the formation of glycine conjugates with 4-aminobenzoic acid or benzoic acid. 6. Male nematodes showed a higher rate of drug metabolism than female nematodes.