The effects of cyclic monomers on the activities of several drug-metabolizing enzymes were evaluated. Female Wistar rats were fed, for 4 wk, a semi-synthetic diet containing different quantities of cyclic monomers isolated from linseed oil heated at 275 degrees C for 12 hr under nitrogen. Microsomal proteins and cytochrome c were significantly increased in rats fed a diet containing 0.1 or 1% cyclic monomers. Aminopyrine demethylation, a model reaction preferentially induced by phenobarbital, was increased by this treatment. NADPH-cytochrome P-450 reductase was also stimulated. Moreover, ethoxyresorufin deethylation, known to be greatly increased by methylcholanthrene-type inducer was only increased threefold by this treatment. The activity of p-nitrophenol UDP-glucuronosyl transferase decreased while the conjugation of bilirubin was stimulated. These results suggest that cyclic monomers isolated from heated linseed oil show some characteristics of phenobarbital-type inducers.