1. A large proportion of a single oral dose of [(14)C]Ionox 220 to rats is eliminated in 24 days: 89.3-97.4% of the label is excreted in the faeces (much of this is eliminated in the first 4 days after dosage), 1% in the urine and less than 0.1% in the expired gases; 4.06% of (14)C is present in the carcass and viscera after removal of the gut, and most of this is in the fatty tissues. 2. About 87% of (14)C in the faeces is due to unchanged antioxidant, 5% to the quinone methide, 5% to the free acid and 3% to an unidentified polar constituent. Three-fifths of (14)C in the urine is due to 3,5-di-tert.-butyl-4-hydroxybenzoic acid and the remainder to the ester glucuronide. In three individual animals, one-half of (14)C in the bile is due to the free acid, one-quarter to the ester glucuronide and the remainder to unchanged antioxidant, whereas in another all of (14)C in the bile is due to Ionox 220. About 97% of (14)C in the body fat is due to unchanged antioxidant and the remainder to the free acid. 3. Up to 20% of a single oral dose of Ionox 220 is absorbed in rats: 13-14% is metabolized. 3,5-Di-tert.-butyl-4-hydroxybenzoic acid accounts for just over 5% of a dose of Ionox 220, 3,5-di-tert.-butyl-4-hydroxybenzoyl-beta-d-glucopyranosiduronic acid for less than 0.4%, the quinone methide for just over 5% and an unidentified compound for less than 3%. 4. The physiological and biochemical implications of ingesting Ionox 220 are discussed.