Cytoplasmic and intranuclear iron depositions were observed in the livers and kidneys of male and female ddY mice treated for 4-12 weeks with ferric nitrilotriacetate (Fe-NTA), a known renal carcinogen that acts through the production of free radicals. The intranuclear iron deposition consisted of a spherical aggregation of ferritin particles of approximately 10 nm diameter, as revealed by immunohistochemical staining and electron microscopy. Although the incidence of renal tumours was greater in the males than in the females, the incidence of iron depositions did not differ with gender. The most abundant intranuclear iron depositions were observed in the animals treated with Fe-NTA for the longest duration (12 weeks). These findings suggest that the intranuclear production and propagation of free radical reactions are prevented by the trapping of iron in a chemically inert iron form of ferritin.