Preliminary studies were undertaken to investigate whether or not added iron (0, 35, 880 ppm of iron as ferrous fumarate) and fat type (corn oil, beef tallow, or menhaden oil ) influenced the oxidation of the rat diet during storage . Iron level affected thiobarbituric acid (TBA) values only in the menhaden oil diets. Storage for 4 d did not affect TBA values of the diets . Neither food intake nor body weight of the rats was affected by the different diets, suggesting all diets were equally acceptable to rats. The effects of iron supplementation and fat type on in vivo lipid peroxidation in rat colon were studied. Semi-synthetic diets were formulated to contain 15% (wt/wt) total fat, an amount comparable with human diets, as either 15% corn oil (C), 1% corn oil + 14% beef tallow (B), or 1% corn oil + 14% menhaden oil (M). Diets of each fat type were formuIated with ferrous fumarate to contain 35 ppm iron, a level sufficient to meet the requirement of the rats, or 880 ppm iron, a level similar to that found in iron-fortified breakfast cereals. During a 6-wk study, each of 6 groups of 10 male weanling Sprague-Dawley rats was fed one of the 6 diets (C35, C880, B35, B880, M35, M880). Lipid peroxidation products in the colon mucosa and in the feces were measured as thiobarbituric acid reactive substances (TBARS), and other possible physiological changes were monitored by measuring body weight, and iron levels of the feces and colon mucosa, and by observing the histology of the colon. At the beginning of the trial, each group of rats had similar body weights and TBARS in the feces. After the feeding trial, groups of rats remained similar in body weight, and no histological changes were observed in the colon. However, rats fed the different dietary fats had different (p < 0.05) TBARS in the feces and colon mucosa (BThus, the type of dietary fat was a significant determinant of in vivo lipid peroxidation, independent of dietary iron level. Rats fed the high iron diets had higher TBARS in both the feces and colon mucosa. When compared by dietary fat type, rats fed high iron diets had higher TBARS in the mucosa only if they were also fed the menhaden oil diet. Thus, dietarv iron was a significant determinant of in vivo lipid peroxidation only in combination with menhaden oil. The long-term intake of iron-fortified foods with high menhaden oil may lead to significant increased in vivo lipid peroxidation.