Iron overload may enhance oxidative damage. Growth hormone (GH) and insulin-like growth factor-I (IGF-I) are involved in oxidative processes, lipid peroxidation (LPO) included. The aim of the study was to evaluate the in vivo effects of GH, IGF-I and/or iron on LPO in rat tissues. Male Wistar rats were administered iron (Fe2+; 3mg/100g b.w., i.p., on the 8th day) and/or GH (0.2IU/100g b.w.), and/or IGF-I (2microg/100g b.w.) once daily for 8 days. LPO products (malondialdehyde+4-hydroxyalkenals) were measured in rat brain, lung, small intestine, liver, kidney, testis, spleen and serum. Iron injection increased LPO only in the small intestine and that effect was completely prevented by either GH or IGF-I. In the brain, GH decreased, whereas IGF-I increased, the basal LPO. GH and IGF-I possess some ability to prevent iron-induced oxidative damage in iron sensitive tissues, but contribute to oxidative imbalance in other tissues.