A number of foreign compounds induce the proliferation of the hepatic smooth endoplasmatic reticulum and thereby increase the activity of monooxygenases that metabolize drugs and other foreign compound. With reference to the safety of food additives some antioxidants have been examined by various authors for their inducing capacity, in doses well above those ingested with treated food and above the stipulated accepted daily intake (ADI). Thus feeding of rats with the very high dose of 500 mg/kg body weight of butylated hydroxtoluene (BHT) resulted in an increase in its own oxidative metabolism. Also in monkeys BHT produces an inductive increase of microsomal enzyme activity, cytochrome p 450, and a proliferation of smooth endoplasmatic reticulum. The closely related antioxidant butylated hydroxyanisole (BHA) leads to similar effects in mice. Recent studies with ethoxyquin (EQ) have shown that this antioxidant stimulates the formation of a form of cytochrome P 450 which resembles the phenobarbital-inducible type. EQ itself is also an inhibitor of mixed function monooxygenase. The naturally occuring flavouring agents safrole and isosafrole in high doses also induce hepatic monooxygenations. Spectroscopic examination has revealed, however, that another type of cytochrome with characteristic binding of a suspected safrole metabolite is produced. This complex shows absorption maxima at 427 and 455 nm and can be dissociated by adding a number of different lipophilic agents, including safrole itself. In a time dependent displacement reaction the characteristic spectrum decreases and is replaced by a classical binding spectrum of the displacer itself. By these examples it can be shown that food additives exert a number of effects on hepatic drug inactivating systems. However, no risk of hepatic changes can be seen from the small amounts of antioxidants ingested in balanced human nutrition.