Iron-induced free radical injuries in male and female ddY mice, especially the sex difference and its mechanisms, were studied after an i.p. injection of a renal carcinogen, ferric nitrilotriacetate. Male mice were much more susceptible to iron-induced free radical injuries than female mice. Oxidative modification of proteins and DNA occurred more strongly in males than in females, as measured by protein carbonyl content and 8-hydroxydeoxyguanosine, respectively. Histochemical detection of 4-hydroxy-2-nonenal-modified proteins using an antibody and DNA fragmentation as detected by the TUNEL method also showed that males are more severely damaged than females, especially in the proximal convoluted tubules. These results could not be explained by the difference in iron status between male and female mice. In fact, the toxic so-called 'free' iron in serum and kidney were not different between male and female mice and storage iron, such as ferritin and hemosiderin, was also comparable in both kidneys. In previous studies we proposed the glutathione cycling hypothesis to explain the sex differences. The half-life of glutathione in the kidney was significantly shorter in males (29 min) than in females (57 min), as determined by the glutathione decrease after buthionine sulfoximine treatment, a specific inhibitor of glutathione synthesis. The specific activity of gamma-glutamyltranspeptidase (EC 2.3.2.2) in female mice was 73% of that in male mice. These results suggest that the faster glutathione turnover in males could account for the higher susceptibility to oxidative injury by supplying the reducing equivalent that reduces Fe(III) to Fe(II), thereby facilitating iron-catalyzed free radical reactions.