Since male A/J mice are much more susceptible to both acute and subacute nephrotoxicity and the carcinogenic effect of ferric nitrilotriacetate than female mice, sex differences in the lipid peroxidation level after ferric nitrilotriacetate use were examined. The effects of orchiectomy and testosterone were also investigated. Male and female A/J mice were given a single intraperitoneal injection of ferric nitrilotriacetate (3 mg of iron/kg of body weight) and then thiobarbituric acid reactivity was determined in the liver and the kidney. Only male mice showed high thiobarbituric acid reactivity after 30 min, with the kidney showing higher activity than the liver. Castrated male mice showed a reduction in thiobarbituric acid reactivity, whereas testosterone-pretreated castrated male or testosterone-pretreated female mice showed increased thiobarbituric acid reactivity. In addition, daily intraperitoneal injections of ferric nitrilotriacetate resulted in the death of all normal male mice within 6 days, whereas all female and castrated male mice survived 3 months of treatment. Thus, male and female mice showed differences in ferric nitrilotriacetate-induced toxicity as reflected in the degree of lipid peroxidation and mortality.