Enhancement of pulmonary epoxide hydratase activity in the rat was obtained by feeding a diet which contained 0.5% ethoxyquin (EQ) or 3,5-di-tert-butyl-4-hydroxytoluene (BHT). The enhancement was less marked (1.6-fold) than in the liver, where a 2-4-fold enhancement of enzyme activity was found after feeding 0.5% antioxidant. A minimal enhancing concentration of 0.1% EQ or BHT in food was established for hepatic epoxide hydratase. In kidney microsomes, elevation of enzyme activity was obtained with 0.5% EQ (1.8-fold), but not with 0.5% BHT. No concomitant increase of cytochrome P-450 content and of aryl hydrocarbon hydroxylase activity was found in the tissues studied.